Chapter 47: The Normal First Trimester

primary yolk sac

first site of formation of red blood cells that will nourish the embryo

secondary yolk sac

formed at 23 days when the primary yolk sac is pinched off by the extraembryonic coelom

double decidual sac sign

interface between the decidua capsularis and the echogenic, highly vascular surface on the opposite side of the endometrial cavity

IUP

intrauterine pregnancy

chorionic villi sampling (CVS)

invasive diagnostic genetic testing that involves sampling zygotic cells from developing placental tissue

menstrual age (gestational age)

length of time calculated from the first day of the last normal menstrual period (LMP) to the point at which the pregnancy is being assessed

The gestational sac or mean sac diameter is determined by calculating the average sum of the ____,____ and ___ of the gestational sac

length, width & height

MSD

mean sac diameter

crown-rump length (CRL)

most accurate measurement of the embryo in the first trimester

monoamniotic

multiple pregnancy with one amniotic sac

monochorionic

multiple pregnancy with one chorionic sac

diamniotic

multiple pregnancy with two amniotic sacs

dichorionic

multiple pregnancy with two chorionic sacs

Monochorionic-diamniotic twins appear to be contained within__chorionic sac;_____amnion,____yolk sac, and ___embryos are identified

one, two, two, two

hematopoiesis

production and development of blood cells

chorionic cavity

surrounds the amniotic cavity; the yolk sac is within the chorionic cavity

zygote

the conceptus resulting from the union of male and female gametes. In a normal pregnancy, the zygote travels through the tubes and implants in the uterus

yolk stalk

the umbilical duct connecting the yolk sac with the embryo

decidua capsularis

the uterine decidua on the surface of the implantation site. As the pregnancy progresses, the decidua capsularis may shed, causing light early pregnancy bleeding

decidua basalis

the uterine decidual surface on the maternal side of the placenta; the decidual basalis is invaded by trophoblastic cells that form the placenta

embryonic period

time between 4 and 10 weeks' gestation

Dichorionic and diamniotic twins appear as ___ seperate gestational sacs with individual trophoblastic tissue, which allows the appearance of a thick dividing membrane

two

Know when the herniation of bowel occurs in the embryo.

The anterior abdominal wall is developed by 6 weeks of gestation from the fusion of four ectomesodermal body folds. Simultaneously, the primitive gut is formed as a result of the incorporation of the dorsal yolk sac into the embryo. The midgut, derived from the primitive gut, develops and forms the majority of the small bowel, cecum, ascending colon, and proximal transverse colon. Because the midgut is in direct communication with the yolk sac, amniotic cavity expansion pulls the yolk sac away from the embryo, forming the yolk stalk. As amniotic expansion occurs, the midgut elongates faster than the embryo is growing, causing the midgut to herniate into the base of the umbilical cord. Until approximately 10 weeks gestation, the midgut loop continues to grow and rotate before it descends into the fetal abdomen at about the 11th week. Sonographically, this transition of bowel within the base of the umbilical cord can be visualized readily. The small bowel appears as an echogenic mass within the base of the umbilical cord; little echogenic bowel is seen within the embryonic or fetal abdomen. After 12 weeks'gestation, the echogenic umbilical cord mass is no longer visualized and echogenic bowel is seen within the fetal abdomen.

Describe what happens to the bowel migration in the first trimester.

The anterior abdominal wall is developed by 6 weeks of gestation from the fusion of four ectomesodermal body folds. Simultaneously, the primitive gut is formed as a result of the incorporation of the dorsal yolk sac into the embryo. The midgut, derived from the primitive gut, develops and forms the majority of the small bowel, cecum, ascending colon, and proximal transverse colon. Since the midgut is in direct communication with the yolk sac, amniotic cavity expansion pulls the yolk sac away from the embryo forming the yolk stalk. As amniotic expansion occurs, the midgut elongates faster than the embryo is growing, causing the midgut to herniate into the base of the umbilical cord. Until approximately 10 weeks gestation, the midgut loop continues to grow and rotate before it descends into the fetal abdomen at about the eleventh week.

Name the embryologic components of the echogenic ring around the gestational sac.

The echogenic ring around the gestational sac can be divided embryologically into several components. The villi on the myometrial or burrowing side of the conceptus villi are known as the decidua basalis, whereas the villi covering the rest of the developing embryo are referred to as the decidua capsularis. The interface between the decidua capsularis and the echogenic, highly vascularized endometrium forms the double decidual sac sign, which has been reported to be a reliable sign of a viable gestation.

When is the amniotic and chorionic cavities routinely imaged with endovaginal ultrasound?

Using EV transducers, the amniotic membrane that separates the amniotic and chorionic cavities is routinely seen after 5.5 weeks. Although with normal gain settings the chorionic cavity (extraembryonic coelom) may appear sonolucent, increased overall gain settings may fill the fluid with low-level echoes, which correspond to a thicker consistency of the chorionic cavity in relation to the amniotic cavity. Later the amniotic cavity expands and the chorionic cavity decreases in size, with eventual chorioamniotic fusion occurring at approximately 16 to 17 weeks.

How closely does visualization of the yolk sac predict a viable pregnancy?

Visualization of the yolk sac predicts a viable pregnancy in over 90% of cases. Conversely, failure to visualize the yolk sac, with a minimum of 8 mm MSD, using EVS, should provoke suspicion of abnormal pregnancy. TA studies have shown that the yolk sac should be seen within mean sac diameters of 10 to 15 mm and always should be visualized with a mean sac diameter of 20 mm.

the fertilized ovum which now should be referred to as a ______, undergoes rapid cellular division in the for of the 16 cell morula

Zygot

embryologic age (conceptual age)

age calculated from when conception occurs

amniotic cavity

cavity in which the fetus exists; it forms early in gestation and fills with amniotic fluid to protect the fetus

the interface btwn the decidua capsularis and the echogenic, highly vascularized endometrium form the ______sign which has been reported as a reliable sign of a viable gestation

double decidual sac

the growth rate of the yolk sac has been reported to be approx. __mm per millimeter of growth of the MSD when the MSD is less than 15mm

0.1

the gestational sac size grows at a predictable rate of ______mm per day in early pregnancy

1

TA studies have shown that the yolk sac should be sean within mean sac diameters of ______mm and should always be visualted with a mean sac diameter of 20mm

10-15

A normal gestational sac can be consistantly demonstrated with the hcg level ranges btwn _________mIU/mL

1000-2000

10 wks gestation the midgut loop continues to grow and rotate before it descends into the fetal abdomen at about the _____wk

11th

the midgut loop of the bowel continues to grow and rotate before it descends into the fetal abdomen at about the __week

11th

failure to see the yolk sac with a minimum of ___mm MSD, using TV sonography could indicate an abnormal pregnancy

12

the embryonic face undergoes significant evolution starting in the 5th wk, with palate fusion beginning around the ___wk of gestation

12th

the early embryo is not seen in TV until the heart motion is detected and the CRL measures approx ________mm

3

the embryonic period is the time btwn__ & 10 weeks gestation

4

The spine is also developing duing th embryonic period, during the ____wks of gestation

5-7th

using TV sonography, multiple gestations can be diagnosed at very early stages between ________wks

5.5-6.5

the primative neural tube closes by ___wks gestation

6

limb buds are embryologically recognizable during the ___wk of gestation

6th

at ___wks, the primary vesicles are seen within the brain: the prosencephalon, the mesenchephalon and the rhombencephalon

8

the cystic rhomboid fossa can sonographically be imaged routinely from the_____ wk of gestation

8-11th

What is the morbidity and mortality rate of a multiple pregnancy in relation to a singleton pregnancy?

A multiple gestation pregnancy is, by definition, high risk, with significant increases in morbidity and mortality rates in relation to singleton pregnancies. Overall, twin gestations have a 7 to 10 time greater mortality rate than singletons.

What level does the hCG need to be to demonstrate a normal gestational sac?

A normal gestational sac can be consistently demonstrated when the hCG level is 1800 mIU/ml (Second International Standard) or greater when using transabdominal sonography. This detection threshold is significantly reduced by endovaginal sonography and may be as low as 500 mIU/ml. Many labs will use hCG levels of 1000 to 2000 mIU/ml as the number to indicate a normal pregnancy.

Describe the relationship between the sonographic findings and quantitative serum hCG levels in early pregnancy.

An intimate relationship between the sonographic findings and quantitative serum hCG levels normally exists during early pregnancy. Gestational sac size and hCG levels increase proportionately until 8 menstrual weeks, at which time the gestational sac is approximately 25 mm MSD and an embryo should be easily detected by either transabdominal or endovaginal sonography. After 8 weeks, hCG levels plateau and subsequently decline, while the gestational sac continues to grow.

What is the sonographic appearance of a dichorionic and diamniotic pregnancy in the first trimester?

Dizygotic (two ova) twin pregnancies, which comprise 70% of all twins, are by definition dichorionic and diamniotic. Sonographically, dichorionic and diamniotic twins appear as two separate gestational sacs with individual trophoblastic tissue, which allows the appearance of a thick dividing membrane. As pregnancy progresses, this membrane becomes thinner secondary to the diminished space between the two sacs, so this diagnosis may be more difficult later in gestation. In a dichorionic-diamniotic pregnancy, each sac has an individual yolk sac, amniotic membrane, and embryo.

Describe the sonographic appearance of an early intrauterine pregnancy.

During the fifth week of embryonic development the intrauterine pregnancy (IUP) can be visualized sonographically. It appears as a 1- to 2-mm sac with an echogenic ring having a sonolucent center. The anechoic center represents the chorionic cavity. The circumferential echogenic rim seen surrounding the gestational sac represents trophoblastic tissue and associated decidual reaction. These include a round or oval shape; a fundal position in the uterus, or an eccentrically placed position in the middle portion of the uterus; smooth contours; a decidua wall thickness greater than 3 mm; and a yolk sac with an MSD greater than 10 mm and an embryo with an MSD greater than 18 mm. The gestational sac size grows at a predictable rate of 1 mm per day in early pregnancy.

the sonographer must be aware that when hcg level is elevated and the gestational sac is not seen in the uterus a _________pregnancy should be considered

Ectopic

in the first 9 menstrual weeks the conceptus is called a_____

Embryo

Describe the normal early cardiac activity.

Embryonic cardiac activity should always be seen by 46 menstrual days. Embryonic cardiac rates vary with gestational age. Rates of 90 bpm at 6 weeks increase to rates of 170 bpm at 9 weeks, with rates of approximately 140 bpm through the remainder of the first and second trimesters.

Name the two parameters for sonographic gestational dating in the first trimester

It is widely accepted that the most accurate gestational dating during pregnancy is the use of ultrasound within the first trimester. Two parameters for sonographic gestational dating may be used: 1. CRL; and 2. gestational sac size.

_____ventricles completely fill the cerebral vault btwn the 8-11th week of gestation

Lateral

At what gestational age is the yolk sac routinely visualized?

Often the first intragestational sac anatomy seen is the sonographic yolk sac (secondary yolk sac), which is routinely visualized at 5 to 5.5 weeks gestation. Invariably, the observer sees the yolk sac before the beating embryonic heart, which may be normal because embryonic heart motion begins at 5.3 to 5.5 weeks. The yolk sac may be used as a landmark to image the embryo, given the connection between yolk sac and embryo.

Can the cranium appear "cystic" during the first trimester?

Once the rhombencephalon divides with its corresponding flexure, the cystic rhomboid fossa forms. The cystic rhomboid fossa can be routinely imaged sonographically from the eighth to 11th week of gestation. With increasing gestational age, further evolution of the cerebellum, medulla, and medulla oblongata encloses the rhomboid fossa to form the primitive 4th ventricle and part of the cerebral aqueduct of Sylvius. This cystic structure, seen within the posterior aspect of the embryonic cranium, should not be confused with pathosis, such as Dandy-Walker deformity.

Monozygotic, monoamntiotic-monochorionic twin gestation shows__gestational sac with ___amniotic membrane, which may contain 1 or 2 yolk sacs and 2 embryos wiht in the single amniotic membrane

One;one

After 8 wks, hcg levels____ and subsequently decline while the gestational sac continues to grow

Plateau

Gestational sac size and HCG levels increase_________until 8 menstrual wks

Proportionally

At what gestational age is the embryo identifiable on ultrasound?

Sonographically, the early embryo is usually not identified until heart motion is detected at approximately 5.5 weeks when the CRL is approximately 2 mm. At this stage the embryo is seen between the secondary yolk sac andimmediate gestational sac wall. Because the amniotic cavity is still relatively small, it appears that no space lies between the yolk sac and embryo.

How is the mean sac diameter determined?

Sonographically, the gestational sac size or mean sac diameter is determined by the average sum of the length, width, and height of the gestational sac. These measurements are obtained in both sagittal and coronal/semicoronal sonographic planes. When measuring the mean sac diameter, the sonographer should only measure gestational sac fluid space, not including the echogenic decidua.

the blastocyst typically enters the uterus 4-5 days after fertilization, with implantation occuring_______days after ovulation.

7-9 days

What is the difference between menstrual age and embryologic age?

All embryonic dates in this chapter reflect menstrual age rather than embryologic age. Menstrual age (also known as gestational age) is calculated by adding 2 weeks (14 days) to embryologic age. Menstrual age refers to length of time calculated from the first day of the last normal menstrual period (LMP) to the point at which the pregnancy is being assessed.

Name the three primary brain vesicles that develop after the sixth week of gestation.

Around the sixth week of gestation, three primary brain vesicles develop: the prosencephalon, mesencephalon, and rhombencephalon. Because of rapid cell proliferation in relation to cranial vault space, flexures of the developing brain occur.

The__________system is the first organ to develop rapidly, with the first heartbeats noted between 5.5-6 wks

Cardiovascular

Understand the difference between transabdominal and endovaginal scanning.

Endovaginal transducers allow the gravid uterus and adnexa to be visualized with greater detail than transabdominal techniques by allowing higher frequencies to be used (5-7.5 MHz) and the placement of the transducer closer to anatomic structures. The transabdominal-transvesical approach allows visualization of a larger field of anatomy, which is important when specific anatomic relationships are in question.

for the time after the first 9 weeks, the embryo is called a ______

Fetus

Clinicians and sonographers use________to date pregnancy, w/the 1st day of the last menstrual period as the beginning of gestation

Gestation

Learn about the goals for sonography in the first trimester

Goals for sonography in the first trimester pregnancy: Visualization and localization of the gestational sac (intrauterine or ectopic pregnancy) Identification of embryonic demise or living embryonic gestation Identification of embryos that are still alive, but at increased risk for embryonic or fetal demise Determination of the number of embryos and the chorionicity and amnionicity in multifetal pregnancies Estimation of the duration or menstrual age of the pregnancy Early diagnosis of fetal anomalies, including identification of embryos that are more likely to be abnormal based on secondary criteria (abnormal yolk sac)

Describe the development of the choriod plexus and cerebral ventricles in the later part of the first trimester.

The cerebral hemispheres may be seen at around 9 weeks' gestation. The echogenic choroid plexus, which fills the lateral cerebral ventricles, can be visualized. Sonolucent cerebrospinal fluid can be demarcated around the choroid plexus. It is important to note that the lateral ventricles completely fill the cerebral vault at this time in gestation. The cerebral hemispheres are relatively small compared with the rest of the brain, although this relationship rapidly changes at the beginning of the second trimester.

the gestational age is calculated by adding ____ weeks to the conceptional age.

Two

human chorionic gonadotropin (hCG)

hormone secreted by the trophoblastic cells (developing placental cells) of the blastocyst; laboratory test indicates pregnancy when values are elevated

the 1st intragestational sac anatomy seen is the sonographic____sac, which is routinely visualized between 5-5.5 wks gestation

yolk

Describe fertilization from the point at which the ovum is swept into the tube to the development of the yolk sac.

The fertilized ovum, which should now be referred to as a zygote, undergoes rapid cellular division to form the 16-cell morula. Further cell proliferation brings the morula to the blastocyst stage, which contains trophoblastic cells and the "inner cell mass," which forms the embryo. The blastocyst typically enters the uterus 4 to 5 days after fertilization, with implantation occurring 7 to 9 days after ovulation. During implantation, proteolytic enzymes produced by the trophoblasts erode endometrial mucosa and maternal capillaries, resulting in a primitive blood exchange network between mother and conceptus. When implantation is completed the trophoblast goes on to form primary villi, which initially circumvent the early gestational sac, giving it the sonographic ring appearance. Within the conceptus the inner cell mass matures into the bilaminar embryonic disc, the future embryo, and the primary yolk sac. At approximately 23 days menstrual age the primary yolk sac is pinched off by the extra embryonic coelom, forming the secondary yolk sac. The secondary yolk sac is the yolk sac seen, sonographically, throughout the first trimester. The amniotic and chorionic cavities also develop and evolve during this period of gestation.

What is the first organ to function in the embryo?

The heart is the first organ to function within the embryo. The embryonic heart starts beating at approximately 35 days (5.3-5.5 weeks) when the endocardial heart tubes fuse to form a single heart tube. Complex embryonic evolution occurs so that by the end of the eighth week of gestation, the heart has obtained its adult configuration.

Describe the development of the rhombencephalon in the first trimester.

The rhombencephalon divides into two segments: the cephalic portion or metencephalon and caudal component or myelencephalon. Once the rhombencephalon divides with its corresponding flexure, the cystic rhomboid fossa forms. The cystic rhomboid fossa can sonographically be imaged routinely from the eighth to 11th week of gestation. With increasing gestational age, further evolution of the cerebellum, medulla, and medulla oblongata encloses the rhomboid fossa to form the primitive fourth ventricle and part of the cerebral aqueduct of Sylvius. This cystic structure, seen within the posterior aspect of the embryonic cranium, should not be confused with pathology, such as Dandy-Walker deformity.

What is the purpose of the secondary yolk sac?

The secondary or sonographic yolk sac has essential functions in embryonic development, including: 1. provision of nutrients to the developing embryo; 2. hematopoiesis; and 3. development of embryonic endoderm, which forms the primitive gut.

When does the fetal skeletal system begin to develop?

The skeletal system begins to develop during the sixth week with the upper limbs forming first, followed by the lower extremities. The hands and feet develop later in the first trimester and are completed by the end of the 10th week of gestation. Sonographically, limb buds can be detected, generally, from the seventh week on; the fingers and toes are recognizable at 11 weeks using EVS.