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Embryology and Anatomy of the First Trimester

Definition of first trimester

• 1st trimester covers time from 1st day of last menstrual period to end of 13th postmenstrual week

gallery case scenario

During the follicular phase of the menstrual cycle, several follicles begin to develop; 1 becomes dominant and eventually a mature oocyte is extruded from the ovarian surface at the time of ovulation. The remaining follicle becomes the corpus luteum, which produces progesterone and helps to maintain the early pregnancy until the placenta is formed. If fertilization does not occur, the corpus luteum degenerates into a corpus albicans. 

The oocyte is swept into the fallopian tube where it is fertilized. The fertilized ovum divides repeatedly during passage along the tube such that by the time it reaches the endometrial cavity, a blastocyst has formed. The blastocyst "hatches" from the zona pellucida and implants into the maternal endometrium. 

While the dividing zygote is still in the fallopian tube (8-cell stage), cells differentiate into embryoblast and trophoblast. Syncytiotrophoblast interacts with the endometrium to form the placenta; the remainder is cytotrophoblast. Embryoblast cells will give rise to the embryo, amnion, and yolk sac 

The embryoblast splits into 2 layers: Epiblast and hypoblast. The hypoblast gives rise to the primary and secondary yolk sacs and extraembryonic mesoderm. The latter splits, forming the chorionic cavity. The epiblast gives rise to the embryo and the amnion 

s the primary yolk sac involutes, the secondary yolk sac develops. It is the secondary yolk sac that is visible sonographically; however, by convention, it is usually referred to as simply the yolk sac on ultrasound images. The chorionic cavity enlarges. The embryo is still a bilaminar disc.

The gestational sac has burrowed into the decidualized endometrium, creating an asymmetrically placed echogenic ring with a lucent center. This was initially described as the intradecidual sac sign (IDSS). It is not always visible in early pregnancy and it is subject to considerable interobserver variability. 

The intradecidual gestational sac is an echogenic ring eccentric to the line created by apposition of the endometrial surfaces. Currently, recommended terms for such an observation are intrauterine sac-like structure or probable intrauterine pregnancy. 

This is an example of the IDSS. In this example, bleeding has resulted in accumulation of blood in the endometrial cavity. Again, note the eccentric location of the 4-mm diameter sac. In the lower image, a tiny circular structure within the gestational sac is likely the primary yolk sac, which can be seen with high-resolution modern transducers. 

This graphic illustrates the double decidual sac sign (DDSS). This is seen when the enlarging gestational sac protrudes from the site of implantation and starts to expand into the uterine cavity, exerting mass effect on the opposite uterine wall. The decidua covering the expanding sac is decidua capsularis; that which is being pushed ahead of the expanding sac is the decidua parietalis. The decidua basalis is where the sac is adherent to the uterine wall and marks the site where the placenta will develop. Internal structures may be seen on transvaginal scans. 

The decidual layers are easily seen on this transvaginal scan. The concentric rings created by the decidua capsularis and parietalis create the DDSS. This finding is characterized as a probable intrauterine pregnancy. 

In this example, the embryo and yolk sac are visible in addition to the DDSS; this indicates a definite intrauterine pregnancy. It would also be a pregnancy of uncertain viability if there was no cardiac activity in an embryo < 7 mm in length. 

The graphic illustrates normal early development. The embryo is intimately associated with the yolk sac such that the amnion and yolk sac appear as a double bleb with the embryo sandwiched between them. The embryo is within the amniotic sac; both the embryo and yolk sac are inside the chorionic sac. 

Vaginal ultrasound at 5 weeks, 5 days from last menstrual period shows a 2-mm embryo. There are 3 linear echoes, which resemble an Oreo cookie. The process of gastrulation results in cellular movement with creation of the 3 primary germ layers; the endoderm, the mesoderm, and the ectoderm. Despite the tiny size of this embryo, cardiac activity was visible in real time. 

Vaginal ultrasound shows the embryo immediately adjacent to the yolk sac. The amnion is not yet visible. At this gestational age, the abdominal wall is still open, and the midgut is in continuity with the yolk sac. After the abdominal wall closes, the "discarded" yolk sac is compressed between the expanding amnion and the chorion. 

Curvature and folding of the embryo result in closure of the abdominal wall and pinching off of the yolk sac. The elongated neck forms the vitelline duct. Eventually the yolk sac separates from the embryo, dropping into the chorionic cavity. At the same time, it becomes clear which end of the embryo is which, and limb buds start to form. The chorion adjacent to the uterine cavity is now completely smooth. Chorionic villi in the developing placenta become more complex in structure.

Vaginal ultrasound at 7 weeks, 4 days from LMP shows the yolk sac is separate from the embryo. It lies outside the amnion, which is now expanded enough to be just visible as it surrounds the embryo. Remember that the yolk sac will always be outside the amnion; the embryo lies inside the amniotic sac.

The graphic illustrates continued embryonic development; the limb buds are evident, the head has grown dramatically, and the embryo is assuming a recognizable human form. The umbilical cord forms as a result of fusion of the vitelline duct, allantois, and connecting stalk. Once formed, it elongates rapidly until the embryo is suspended within the enlarging amniotic sac. Cord elongation allows for free mobility of the developing fetus. 

3D surface-rendered ultrasound of an 8-week embryo show the short limb buds and the relatively thick umbilical cord. The crown end is assuming a more recognizable head shape and the embryo is curling into the typical fetal position.
Toward the end of the 1st trimester, the amnion fills the chorionic cavity. The membranes do not "fuse" until 14-16 weeks. The placenta continues to grow, and the chorionic villi develop an increasingly complex branching pattern 

Sagittal transabdominal ultrasound at 12 weeks, 6 days from LMP shows recognizable anatomic features. Organogenesis is completed by the end of the 13th week. Growth and maturation of the various organ systems occurs during the remainder of gestation. 
3D surface-rendered ultrasound at 10 weeks shows increasingly recognizable fetal anatomy with a well-developed cranium and extremities. The abdominal wall cord insertion site is quite broad due to the physiologic herniation of bowel into its base. This occurs as the peritoneal cavity is too small to accommodate the rapidly growing bowel at this gestational age. 

The graphic illustrates herniation of bowel into the base of the cord in the 1st trimester. This happens as the gut tube elongates before there is adequate room to accommodate it in the peritoneal cavity. The gut undergoes a 90° counterclockwise rotation as it herniates and an additional 180° rotation as it is retracted into the peritoneal cavity. Only the gut herniates; liver is never normally seen at the base of the cord. 

Vaginal ultrasound at 10 weeks, 3 days shows echogenic bowel at the base of the umbilical cord. This is normal at this gestational age and should not be misinterpreted as a bowel-containing omphalocele . 

3D surface-rendered ultrasound of a 12-week fetus shows a normal abdominal wall contour. The cord insertion is normal, and there is no residual bowel herniation. Three of the extremities are seen, the cranial contour is normal, and the cord is already coiled. 

The graphic illustrates fetal circulation in which oxygenated blood from the placenta returns to the fetus via the umbilical vein.The umbilical vein courses through the left lobe of the liver to the left portal vein, across the ductus venosus into the inferior vena cava. The umbilical cord also contains 2 arteries, which arise from the internal iliac arteries. 

At 10 weeks, there is some residual herniation of bowel into the base of the cord. The embryo is freely suspended within the amniotic sac by the cord, which already shows evidence of coiling. The yolk sac will be obliterated as the amnion apposes to the chorion. 

Color Doppler ultrasound shows a small umbilical cord cyst as an avascular area adjacent to the cord vessels. As a 1st-trimester finding this is usually of no significance; the cysts form at 8-9 weeks and usually resolve by 12 weeks 

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