The Normal First Trimester Fetus

Several ultrasound studies using mainly the transvaginal approach at 4 to 12 weeks of gestation describe the developent of the normal fetus as fallows :

The 4th Week:
The gestational sac is visible from about 4 weeks and 3 days as a hypoechoic , ring-like2 to 3 mm structure surrounded by a hyperechoic rim . The sac represents the chorionic cavity and is typically located in the upper part of the decidualized endometrium in an eccentric position .

The 5th Week:
The yolk sac becomes visible at 5 weeks . At the end of the 5th week , the fetal pole measures 2 to 3 mm ; we can also identify the yolk sac and the heart activity . The fetal heart rate is about 110 beats per minute .

The 6th Week:
The embryo measures 4 to 8 mm (CRL=crown-rump length) . We identify the fetal pole , the yolk sac and the heart activity . The heart activity should always be seen in embryos of 5 mm ( 6 weeks and 4 days) .
The gestational sac contains two fluid-filled cavities: the celomic cavity (chorionic) with the yolk sac ; the amniotic cavity who is smaller and contains the fetal pole . The two cavities are separated by a thin membrane surrounding the embryo (is the amniotic membrane ) .
At the end of the 6th week , the cavity of the rhombencephalon becomes visible as a small hypoechogenic area at the cephalic part of the fetal pole .

The 7th Week:
CRL is 9 to 14mm . The heart rate is about 130 beats per minute . The amniotic cavity starts expanding rapidly .
The head can be distinguished from the body and hypoechogenic area (brain vesicles) appear in the developing brain at the cephalic end of the embryo . At this stage , the rhombencephalon is larger than the developing hemispheres . The small cavities of the hemispheres originating from the dividing Y-shaped telencephalon , the diencephalon ( who’ll form the thalami , hypothalamus and the 3rd ventricle ) and the mesencephalon (that will form the nuclei and the aqueduct of Sylvius ) may be identifiable .
The umbilical cord is short and appears wider and hyperechogenic at the point of the insertion in the abdomen ; this is the first sign of bowel herniation into the cord . The lower limb buds can vaguely be depicted .

The 8th Week:
CRL is 15 to 22 mm . The brain vesicles are more proeminent and the choroid plexus can be visualized as small hyperechogenic areas . The rhombencephalon is still the largest cavity lying on top of the brain . There is a connection between the lateral ventricles , the diencephalon and the mesencephalon .
The fetal heart rate is about 160 beats per minute and the heart occupies more than 50% of the thoracic cavity . Atria and ventricles are sometimes visible at the end of the 8th week .
The abdominal cavity is fully occupied by the liver anteriorly and the stomach dorsally while the intestine is herniated into the umbilical cord . Fluid in the stomach can occasionally be seen toward the end of the 8th week probably as a product of the gastric epithelium . (The swallowing movements starts just at 11 weeks ) .
The outlines of the skull , spine and ribs can vaguely be seen . The limbs appear as short echogenic outgrowth . The first body movements become visible during the 8th week .

The 9th week:
CRL is 23 to 31 mm . The amniotic cavity is larger than the celomic cavity and occupies most of the sac volume . The fetal body becomes ellipsoid with the head being disproportionally big and the soles of the feet touch in the the midline .
The choroid plexus are obvious and occupy almost fully the lateral ventricles . The cortex is thin and hypoechoic . The cerebral hemispheres become visible and are clearly separated . The connection between the mesencephalon and the third ventricle becomes narrower .
The fetal heart rate reaches a peak of about 175 beats per minute .
Bowel herniation is visible in all fetuses .
The first ossification centres can be seen at the mandible and clavicle .
Limb movements can be identified .