Ch. 28: Pregnancy and Human Development

Neurulation and early mesodermal differentiation

1) 17 days. The flat 3-layered embryo has completed gastrulation. Notochord and neural plate are present 2) 20 days. The neural folds from by-folding of the neural plate, which then deepens, producing the neural groove. Three mesodermal aggregates form on each side of the notochord (somite, intermediate mesoderm, and lateral plate mesoderm) 3) 22 days. The neural folds have closed, forming the neural tube which has detached from the surface ectoderm and lies between the surface ectoderm and the notochord. Embryonic body is beginning to undercut. 4) End of week 4. Embryo undercutting is complete. Somites have subdivided into sclerotome, myotome, and dermatome, which form vertebrae, skeletal muscles, and dermis respectively. Body coelom present.

Forms of contraception

1) Abstinence (no sex) 2) Sterilization (tubal ligation or vasectomy; when a doctor snips, blocks, burns, or ties up the fallopian tubes or vas deferens, effectively ending the gamete express by permanently preventing the sperm or egg from ever getting to the penis/uterus) 3) Barrier methods (work by physically keeping sperm/egg on either side of a wall which is usually made of latex 4) Hormonal methods (the pill/shots/patches; use synthetic estrogen and progestin to prevent the progression of the ovarian cycle to ovulation, and sometimes make changes to the functional uterine layer, the endometrium and cervical mucus to help deter sperm) 5) IUD (Intrauterine divide; a small t-shaped object a medical provider inserts into the uterus to prevent zygotes from implanting.

Events of fertilization

1) After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. 2) Sperm and ovum nuclei swell, forming pronuclei 3) Pronuclei approach each other and mitotic spindle forms between them 4) Chromosomes of the pronuclei intermix. Fertilization is accomplished. Then, the DNA replicates in preparation for the first cleavage division

*Sperm use acrosomal enzymes and receptors to approach, bind, and enter the oocyte. The cortical reaction prevents further sperm entry, ensuring that only 2 copies of each chromosome are present in the fertilized ovum

1) Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulose cells of the corona radiata 2) Acrosomal reaction. Binding of the sperm to sperm-binding receipts in the zone pellucid causes the Ca2+ levels within the sperm to rise, triggering the acrosomal reaction. Acrosomal enzymes from many sperm digest holes through the zone pellucid, clearing a path to the oocyte membrane. 3) Binding. The sperm's membrane binds to the oocyte's sperm-binding receptors. 4) Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 5) Block of polyspermy. Entry of the sperm's contents causes Ca2+ levels in the oocytes cytoplasm to rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucid hardens and the sperm receptors are clipped off (slow block to polyspermy).

Relative size of the uterus before conception and during pregnancy

1) Before conception: Uterus the size of a fist and resides in the pelvis 2) 4 months: funds of the uterus is halfway between the pubic symphysis and the umbilicus 3) 7 months: funds is well above the umbilicus 4) 9 months: funds reaches the xiphoid process

Cleavage stages: From zygote to blastocyst

1) Zygote (fertilized egg) 2) 4-cell stage: 2 days 3) Morula (a solid ball of blastomeres). 3 days 4) Early blastocyst (morula hollows out, fills with fluid, and "hatches" from the zone pellucida). 4 days. 5) Implanting blastocyst: consists of a sphere of trophoblast cells and an eccentric cell cluster called the inner cell mass. (7 days)

Expulsion Stage

Actual childbirth stage, forceful contractions force fetus from the uterus into vagina *Baby's head extends as it is delivered *Any interference with oxygen and prostaglandins is an interference with labor because contractions stop *Breach birth: when baby is coming out feet first. Usually leads to c-section

Placental Stage

Aka afterbirth. Placenta separates from the uterine wall and is expelled. Uterine contractions continue (because after you birth the child you still have to push out the placenta) *After baby is delivered, the placenta detaches and is removed

Resolution phase

As the body relaxes after all the excitement, it enters the resolution phase. Here we see males entering a refractory period that may last several minutes to a day or two, during which they are usually unable to reach orgasm again. Anatomical females are thought not to need this time out physiologically- though they may need it physically.

The sperm that do make it to the uterus, and then the fallopian tube, have another hurdle to overcome, because they can't yet actually penetrate an oocyte. They have to be "capacitated" before they can make a move.

As they swim through the cervix, uterus, and fallopian tube, female secretions start to degrade some of the sperm's protective proteins until the cap on the tip of its little head- called the acrosome- is fragile enough it can leak special hydrolytic enzymes.

Why do the testes dangle outside the abdominal cavity, in the scrotum?

Because sperm are touchy when it comes to temperature. The rate at which they divide, copy DNA, and even feel the effects of some hormones, are all diminished at the core body temperature of 37 degrees. Therefore, the testes have to be outside of the body cavity to reach the lower temperature necessary for proper spermatogenesis (sperm production)

Implantation

Begins 6 - 7 days after ovulation Trophoblasts form 2 distinct layers Burrows into lining Completed by 12th day after ovulation Corpus luteum maintained by hCG

Germ Layers (Part of Embryonic Development)

Blastocyst begins conversion to gastrula 3 primary layers: Endoderm, Ectoderm, and Mesoderm *Ectoderm becomes amnion; gives rise to nervous system and lining of mouth and anal canal *Endoderm becomes yolk sac; forms digestive/respiratory tract *Mesoderm: Internal organs (?) like kidneys, etc.

Dilation Stage

Cervix thins and softens. Regular contractions, amnionic sac ruptures ("my water broke") *Early dilation: baby's head engaged; widest dimension is along left-right axis *Late dilation: (this is the longest part. Walking helps speed it up bc of gravity. Baby's head rotates so widest dimension is in anteroposterior axis (of pelvic outlet). Dilation nearly complete

Fetus

Conceptus (week 9) through birth

Blastocyst

Contains a single outer layer of large, flat trophoblast cells, and inside a cluster of smaller cells forms, called the inner cell mass. This mass is what's going to turn into the embryo eventually while the trophoblasts will go on to form the placenta and blood vessels that will nourish it.

Conceptus

Developing offspring

Epiblast--> Ectoderm

Epidermis, hair, nails, glands of skin Brain and spinal cord Neural crest and derivatives (ex. cranial, spinal, and sympathetic ganglia and associated nerves; chromaffin cells of the adrenal medulla; pigment cells of the skin)

Epiblast--> Endoderm

Epithelial lining and glands of digestive and respiratory tracts

17 - 20 Weeks

Fetal position

Development of fetal circulation

First blood cells in yolk sac Unique vessel modifications : -Umbilical arteries and umbilical vein -All occluded at birth Vascular shunts - Ductus venosus, Foramen ovale, Ductus arteriosus Where do they go, what do they do?? *Umbilical vein is carrying freshly oxygenated blood

Placentation

Formation of placenta from embryonic and maternal tissues -Temporary organ -Embryonic tissues -Maternal portion of placenta - Decidua basalis -Fetal portion of placenta - Chorionic villi (Fully formed and functional by end of third month, nutritive, respiratory, excretory, endocrine functions) *If inadequate, pregnancy aborted (miscarriage)

Yolk sac (Part of Embryonic Development)

Forms part of digestive tract

Chorion (Part of Embryonic Development)

Forms placenta

Embryo

From fertilization through week 8

Sperm leaves seminiferous tubules that made them once mature. In order to have energy to get going however, they need a bit more help than just the tail. Each tubule is surrounded by several layers of hyoid cells, which, like smooth muscles, rhythmically contract, using peristalsis to squeeze sperm- and some fluid secreted by the sertoli cells- through all the twists and turns, toward their next destination, the rate testis in the posterior testis.

From there, the sperm- although still immobile- leave the testes, by way of the epididymis, a long, tangled set of tubes behind the testes where they'll spend the next few weeks gaining their mobility.

13 - 16 Weeks

Glands, face (human), bones

When puberty comes a-knocking, the hypothalamus starts releasing gonadotropin-releasing hormone, of GnRH.

GnRH tells the anterior pituitary to secrete follicle-stimulating hormone and luteinizing hormone into the blood, just like in females. But in females, LH leads to the release of estrogen in the follicles, whereas in males, it spurs the Leydig cells to release testosterone

8 Weeks

Head as large as body, all body systems

Human chorionic gonadotropin (hCG)

Hormonal change during pregnancy. Secreted by trophoblast cells Promotes placental development Levels rise until end of 2nd month, then decline as placenta secretes progesterone and estrogen *When you get pregnant, the hormone gonadotrophin skyrockets

Soon however, it's going to need permanent nutritional support. So, about a week after ovulation, it snuggles up to the endometrial layer and starts the process of implantation.

Implantation is only possible thanks to surges of estrogen and progesterone from the corpus luteum- the ruptured follicle. Together they prepare the endometrium to receive the blastocyst, allowing the uterine lining to bind to little proteins on the trophoblasts holding onto them for the duration of the pregnancy.

The process by which a spermatid elongates, grows a tail, or flagellum, and officially becomes a mobile sperm is called spermiogenesis. The whole process takes about 5 weeks.

In the end, each primary spermatocyte gives rise to 4 actual sperm. And considering how many spermatogonia are continuously dividing into spermatocytes, it's easy to see how a mature male can crank out 1500 sperm a second.

Spermatogenesis is a pretty easy process biologically. It's kind of like how your skin cells are produced- with stem cells at the basal layer that produce immature cells, which in turn get forced toward the surface as they mature

In this case though, the sperm develop on the outer edge of the tubule, and progress inward toward the lumen. It all begins with a trigger (a hormonal cascade)

Reproductive technology

In vitro fertilization - oocytes and sperm incubated, embryo implanted in uterus Zygote intrafallopian transfer (ZIFT) - fertilized oocytes to uterine tubes. Gamete intrafallopian transfer (GIFT) - sperm and oocytes transferred to uterine tubes Cloning -- Issues

21 - 30 Weeks

Increase in weight, eyes are open

Now estrogen takes over. This is partially because the fetus itself is ready to go and it starts releasing hormones like cortisol, that tell the placenta to release even more estrogen to get the uterus ready for birth

Just like hCG calls the shots around the time of implantation, here estrogen is barking out all kinds of orders. For one thing, it prepares the uterus to start receiving new chemical signals, by triggering its myometrial cells to start making receptors for the hormone oxytocin

Epiblast--> Mesoderm--> Intermediate mesoderm

Kidneys Gonads

Gestation Period

Last menstrual period until birth (280 days)

Expulsion stage

Lasts from full dilation through crowning and actual delivery as the infant is pushed head first through the cervix and out of the vagina

Effects of Pregnancy on the Mother

Many changes take place in the mother's organ systems during pregnancy. Reproductive organs engorged with blood. Lordosis common due to the change of the body's center of gravity. (Lordosis is a curving inward of the lower back) Hormone, relaxin, causes pelvic ligaments and pubic symphysis to relax. (so that you'll have room to push out the baby) Weight gain about 28 pounds. Good nutrition - crucial GI tract - morning sickness due to elevated levels of estrogen and progesterone. *Lordosis: a curving inward of the lower back **Big problem with premature babies: underdeveloped lungs (no surfactin cell which holds open alveoli)

Plateau phase

Marked by increased muscle tension, heart rate, blood pressure, and breathing rates. Here the male urethral sphincter contracts to make sure urine doesn't mix with semen.

The oocyte then quickly completes meiosis II, leaving 2 daughter cells: the large ovum and a puny 2nd polar body

Meanwhile, the sperm's detached nucleus swells up to 5 times its original size, forming the male pronucleus, one of 2 sort of proto-nuclei that contain each gamete's genetic information. The other one, the female pronucleus, takes shape from the nucleus of the ovum, and then the 2 start to approach each other, pulled along a protein scaffolding (mitotic spindle) between the 2.

Epiblast--> Mesoderm--> Notochord

Nucleus pulpous of intervertebral discs

Gastrulation

Occurs week 3 3 layered embryo with primary germ layers Sets stage for organogenesis Week 8- all organ systems recognized; End of embryonic period

Cleavage

Occurs while zygote moves toward uterus Mitotic divisions of zygote - 72 hours, morula Blastocyst (Days 4/5) reaches uterus Fluid filled hollow sphere with trophoblast cells, inner cell mass

These hydrolytic enzymes are the key to getting through the oocyte's protective outer layers. Before this however, it has to weave through the granulose cells that form the oocyte's outermost layer, known as the corona radiate.

Once it's past that, it runs into the zone pellucida, a layer of glycoprotein that covers the oocyte's plasma membrane

It takes sperm nearly 20 days to work their way through this labyrinth, during which time they continue to mature

Once through the duct, they enter the inferior epididymis, where they gain mitochondria so they'll have energy to swim hard at a moment's notice. At this point they have the hardware to swim, but still lack the ability to actually move on their own, a skill kept under wraps until they leave the epididymis and get activated by a series of glandular secretions.

Up until the last few weeks before birth, the placenta has been kicking out equally high amounts of both progesterone and estrogen

One of progesterone's main jobs has been to keep the smooth muscles in the uterus relaxed so they can't contract and stimulate labor too early. But as the due date nears, the mother undergoes a sudden decline in progesterone

Throughout childhood, a spermatogonia cell uses mitosis to continuously divide into 2 identical daughter cells. But when puberty starts, the testosterone causes them to divide differently- instead of splitting into 2 identical spermatogonia cells, they begin producing 2 distinct spermatogonia

One type of daughter cell- known as a type. A cell- stays up near the basal lamina and just keeps dividing, so you never run out of spermatogonia. But the other kind- the type B's- get pushed down the tubule toward the lumen, and turn into primary spermatocytes.

From egg to zygote

Oocyte: viable for 12 to 24 hours Sperm: viable for 24 to 48 hours after ejaculation For fertilization: coitus, 2 days before ovulation, 24 hours after ovulation

Around the end of week 8, the embryo becomes a fetus.

Over the next several months, it rapidly develops organ systems and bones and grows into a baby. The growing placenta is still secreting estrogen and progesterone, but its also pumping out relaxin, a hormone that loosens joints and ligaments to increase flexibility and it also releases human placental lactogen (hPL)

Epiblast--> Mesoderm--> Lateral plate mesoderm--> Somatic mesoderm

Parietal serosa Dermis of ventral body region Connective tissues of limbs (bones, joints, and ligaments)

Bilbo-Urethral glands

Pea-sized glands below the prostate, that secrete a clear mucus that drains into the urethra to clear out any acidic urine prior to ejeculation

Orgasm phase

Pelvic and other muscles around the body contract rhythmically as an anatomical male ejaculates, releasing the sperm that might just get a chance to fertilize an egg. The resulting uterine contractions and increased lubrication of a women's orgasm can help draw up and hold sperm in the uterus

An ovum and follicle take over a year to develop, and, as human cells go, it's huge- just large enough to be seen with the naked eye.

Plus, it requires a whole network of supporting tissues in the uterus while it waits for fertilization which- if that doesn't happen- then have to be shed and re-built a month later

Lactation

Production of milk by mammary glands Colostrum - Less lactose, more protein, Vitamin A, almost no fat, rich in IgA antibodies, followed by true milk Prolactin lessens after birth, lactation sustained by nursing Advantages of breast milk - Breast milk is the best milk

Once a path is cleared, a lucky sperm can wiggle against the now-exposed oocyte membrane until it locks onto another set of sperm-binding receptors. And when a sperm docks into one of those, the membrane of the sperm fuses with the innermost membrane of the oocyte, and the contents of the sperm enter the oocyte proper.

Remember that the "egg" is still just a secondary oocyte here, and hasn't completed meiosis. As the sperm delivers its payload, it causes a flash of calcium ions from the oocytes endoplasmic reticulum. This tells the secondary oocyte to get ready to complete its second meiotic division. But it also triggers a reaction that suddenly seals the egg by destroying all the leftover sperm receptors in the membrane, preventing any other sperm from weaseling in.

The testes (male gonads)

Responsible for making male gametes, sperm, and the androgen hormone testosterone

Epiblast--> Mesoderm--> Somite

Sclerotome: vertebrae and ribs Dermatome: dermis of dorsal body region Myotome: trunk and limb musculature

Excitement phase

Sexual arousion triggers the parasympathetic reflex that causes arterioles in the external genitalia to dilate and fill with blood, which is actually one of the only times the parasympathetic system controls any arteriole action

9 - 12 Weeks

Skin, facial features, sex determined

During ovulation, about once a month, an ovary releases a secondary oocyte that heads down the fallopian tube and toward the uterus. That oocyte has a window of about a day or less to meet a sperm before it's no longer viable.

Sperm can persist in the female reproductive tract for as long as 3-5 days

Accomplishing fertilizaiton

Sperm motile, must be capacitated Secretions of female tract weaken Sperm follow "olfactory trail" Sperm must breach oocyte coverings Sperm head approaches oocyte, membranes fuse Only one sperm allowed in (monospermy)

Completion of Meiosis II and Fertilization

Sperm nucleus moves to oocyte nucleus forms male pronucleus Ovum nucleus swells to become female pronuclear Fertilization: movement when 2 pronuclei rupture and chromosomes combine

Fertilization

Sperm's chromosomes combine with secondary oocyte to form zygote

The postnatal period

The 6 week period following birth Baby adjusts to outside of the uterus Liver of newborn is immature, kidneys immature to concentrate urine Body temperature unstable Umbilical vessels constrict and shunts close

Dilation period

The earliest stage of labor, when the first contractions begin, to when the cervix becomes fully dilated to about 10 centimeters. During this time, each new contraction pushes the infant's head against the cervix, causing the cervix to thin and dilate. Once the cervix is fully dilated, the mother should feel the urge to push.

Prostate gland

The enzyme PSA makes the fluid more fluid (easier to move through) and antioxidant amine spermine (which is present in all eukaryotic cells to protect DNA) also somehow increases motility by acting within the sperm. Incidentally, it also inhibits motility at high doses, gives semen a characteristic odor, and is why some people claim semen is good for skin.

Events of fetal development

The fetal period begins at the end of the 8th week and ends at birth. During this period, the growth of the offspring is rapid.

Neonatal period

The first 4 weeks of the postnatal period when the baby is considered a neonate

When that time comes, during ejaculation, the sperm flow from the epididymis through the vas deferens, a tube that travels up behind the bladder and joins with the duct from the seminal gland to create the ejaculatory duct.

The left and right ejactulatory ducts pass into the prostate gland where they empty into the urethra, which runs from the bladder, through the penis, and into the outside world

About 3 days after fertilization, these divisions have formed a little berry-shaped cluster of cells that looks different and complex enough to get a whole new name- a morula (latin: "mulberry"), marks the end of the cleavage stage.

The little cells that make up the morula don't stay together as a solid mass, instead they start to form a hollow sphere filled with fluid- a blastocyst.

hCG is basically calling all the shots, triggering the release of other hormones that are crucial to the blastocyst's development. While hormones are doing the grunt work, the blastocyst is really in charge now. Essentially it has taken over hormonal control of the whole uterus.

The placenta is forming as this happens which is going to eventually take over the hormone-controlling job. Organ that only appears during pregnancy, and is created by the melding of both maternal and embryonic tissues.

This system of tubes feeding into tubes allows all of the necessary glands to make their contributions to a moving wave of seminal fluid that helps sperm mature and perform their ultimate function- fertilization

The resulting mix of sperm, testicular fluid, and gland secretions- which we call semen- provide sperm with transportation, nutritional energy, chemical protection, and finally activates their motility

Meanwhile, the FSH triggers the sertoli cells to release androgen-binding protein, or ABP. This binds to the testosterone, creating large, local concentrations of the stuff, which is ultimately what triggers the production of sperm

The targets of the testosterone are the outermost cells in the tubules, called spermatogonia. They're the stem cells that set the sperm-making process in motion, by dividing. And actually, they've been doing that all along- even before puberty.

These primary spermatocytes then go into meiosis I, and form 2 smaller haploid cells called secondary spermatocytes.

Then they rush through meiosis II, and their resulting daughter cells total 4 round spermatids. These spermatids now have all of the 23 chromosomes they need for fertilization. But they aren't exactly mobile. If they're ever gonna find themselves an egg, they'll need a way to get around- they need a tail.

It also triggers the formation of gap junctions between smooth muscle cells in the uterus- this will let individual muscle cells contract simultaneously when the time comes

Then, as labor nears, special cells in the fetus itself start secreting oxytocin, which binds to all the newly-minted receptors and tells the placenta to release prostaglandins. Together, both of these hormones- oxytocin and prostaglandins- stimulate the uterine muscles to start contracting

Among the most important are the sertoli cells inside the tubes, which nourish developing sperm cells, sort of like how female follicle cells help oocytes.

There are also Leydig cells that secrete testosterone, much like the corpus luteum that secretes estrogen

This is where it starts to get a warmer reception: this layer has specific sperm receptors waiting for it to bind to, and when it does, it opens a bunch of calcium channels, which flood the sperm with calcium ions.

These calcium ions trigger the final acrosomal reaction that releases the enzymes the sperm needs to work through the zone pellucida.

The placenta and umbilical cord provide the direct transfer of nutrients, hormones, and wastes between mother and offspring.

These elements start to really take shape after implantation is complete and we enter the embryonic stage. This is where the blastocyst differentiates into various cell types and develops into a legit embryo, surrounded by an amniotic sac, and hooked up to the placenta.

Each teste is divided into about 250 sections, called lobules, which are loaded with tightly-coiled seminiferous tubules

These tubules are the sperm factories, made of a stratified epithelium surrounding a central fluid-filled lumen. This sperm-making work is supported by cells throughout and around the tubules

The bulk of the epididymis consists of the enormous duct of the epididymis, which, believe it or not, could be uncoiled to stretch about 6 meters- though I don't suggest doing that.

This duct is full of tiny microvilli called stereocilia, which provide a huge surface area to help reabsorb some extra fluid, and help pass along nutrients to feed the idling sperm.

The seminal vesicles

This fluid makes up about 70% of semen. It's slightly alkaline to help counteract the slightly acid environment both in the male urethra and the female vagina. A pair of small, hollow glands behind the bladder that secrete a yellowish, slightly alkaline fluid that contains coagulating enzymes, fructose, and other things to help nourish and transport sperm.

This hormone cocktail tells the fetus to grow and get the breasts ready to lactate and it also causes the mother's body to start hoarding glucose for the fetus to use.

This increase in metabolism, combined with the fact that the kidneys also have to process waste from the fetus, leads to greater urine production, which every expectant mother is familiar with.

Also has a significant effect on the cardiovascular system because the blood volume can increase by as much as 40% (2 liters).

This increased blood flow and pressure can actually make your gums swell and bleed and fluid retention can literally change the shape of your corneas, potentially blurring your vision. Also the expanded uterus compresses pelvic blood vessels, affecting veins' ability to bring blood up from the lower limbs, resulting in swelling, varicose veins, and hemorrhoids.

Amnion (Part of Embryonic Development)

Transparent sac filled with amniotic fluid; regulates temperature. (The fluid allows movement of growth so that growing body parts don't get stuck together)

If all goes according to plan, implantation takes about 5 days and finishes up around around 12 days after ovulation, right around when menstruation would otherwise kick in and slough off the endometrium.

Trophoblasts secrete a luteinizing-like hormone called human chorionic gonadotropin (hCG). This hormone bypasses the whole hypothalamic-pituitary-ovarian-axis and talks directly to the corpus luteum, telling it to keep pumping out estrogen and progesterone.

Allantois (Part of Embryonic Development)

Umbilical cord, urinary bladder

Effects of pregnancy on the mother cont.

Urinary system - urine production increases to handle additional fetal wastes. Respiratory system - edematous and nasal congestion Cardiovascular system - blood volume increases 25 - 40%. Parathyroid hormone levels are high, positive calcium balance. Homeostatic Imbalance - Preeclampsia (A potentially dangerous pregnancy complication characterized by high blood pressure; leads to edema--> swollen ankles) *Edematous: an accumulation of an excessive amount of watery fluid in cells, tissues, or body cavities

Epiblast--> Mesoderm--> Lateral plate mesoderm--> Splanchnic mesoderm

Wall of digestive and respiratory tracts (except epithelial lining) Visceral serosa Heart Blood vessels

1st step of a zygote: divide aka cleavage stage

When cells cleave / split in 2 over and over. Starts about 24 hours after fertilization, when a little zygote turns from 1 cell into 16 cells called blastomeres. The cells divide so quickly that they don't actually grow between divisions- they just create more smaller cells. This allows each little cell to have more surface area, which helps them take in the oxygen and nutrients they need from their environment. Also contains a lot of raw materials that are necessary for building an even larger zygote and eventually a little embryonic human

The birth process

When progesterone levels fall and uterine contractions are no longer inhibited, the birth process begins. The uterus secretes prostaglandins that stimulate uterine contractions, which cause the pituitary gland to secrete oxytocin. Oxytocin stimulates stronger and stronger contractions until the baby is born.

During all of these early divisions, the zygote, then the morula are on the move- they're headed down the fallopian tube toward the uterus.

When the blastocyst reaches the uterus, it just floats around for a few days, soaking up secretions that are full of vitamins and glycoproteins, looking for a place to call home.

Placental stage

Within about 30 minutes of delivery, strong contractions carry out the placental stage of labor, dislodging the placenta from the uterine wall to deliver the so-called afterbirth