A fruitful obstetrics study can never be done if the obstetrics timetable or happenings are not appreciated by the operator.
Sonoembryology Calendar in Brief
Ovulation, in a natural cycle, takes place 14 days prior to the next cycle. Fertilization takes place within approximately 24 hours of ovulation to form the zygote which gets completely implanted, by D-23 of an approximately 4 weeks cycle. From here till the 10th week it is known as the embryo.
The endometrium that had been proliferating under the endocrine influence of the cycle, becomes secretory and embedding blastocyst burrows through this endometrium which then gets sealed off. The fluid-filled cavity of the blastocyst cavity of the embryo is visualized as the anechoic sac at the 4th week. As the extraembryonic coelom developed the primary yolk sac is pinched off and extruded resulting in the formation of the secondary yolk sac which is actually visible on ultrasound around the 5th week with transvaginal scanning (TVUS).
At times the yolk sac is only seen and the foetal pole is not yet visualized. The yolk sac has a diameter of 3 to 6 mm throughout its presence. It is essential to study the yolk sac, measures its diameter and wall thickness. It has been reported that enhanced thickening of the wall has been associated with a congenitally anomalous foetus at later gestational age. Sometimes too thin a wall has been reported to be associated with spontaneous abortion. In the presence of a healthy yolk sac, even when the foetal pole is not demonstrable, further studies have revealed a healthy growing foetus.
Towards the end of the 5th week of gestational age, the formation of the neural plate and neural tube takes place. From the cephalic end of the tube brain vesicle formation starters towards this time. Failure of completion of the neural tube causes defornities with a resistant increment of maternal serum alpha-fetoprotein.
Alpha-Fetoprotein (AFP)
First detected in human foetal serum in 1956, it has a molecular weight of 68,000 daltons and is, therefore, small enough to cross the placenta. It is normally virtually undetectable in the adult except in the presence of certain types of the tumour but is detectable in foetal serum and is at a peak approximately at 13-14 weeks of gestation (3 mg/ml), thereafter falling rapidly at birth (cord serm). Amniotic fluid concentration follows a similar pattern but is 100-200 times less per unit volume, where its chief source is foetal urine in mid-trimester. But in contrast, the maternal serum AFP rises in concentration throughout pregnancy reaching a peak at 30 weeks. At 16 weeks the concentration is about 30 mg/ml.
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