A 29-year-old pregnant woman presents to the obstetrician for prenatal care. She is currently at 26 weeks’ gestation, and this is her first prenatal visit. Her pregnancy has been uneventful so far. Past medical history is unremarkable, and she does not take any medications. The in-office ultrasound indicates the presence of two anatomically separate fetuses with individual amnions. Further study indicates the twins are sharing a placental blood supply.
C) Monozygotic, split between days 4 and 8
This patient is pregnant with monochorionic, diamniotic twins. This is the most common type of monozygotic twinning. For this to occur, splitting must occur between the morula stage (day 4, when the chorion is established) and blastocyst stage (day 8, when the amnion is established).
Answer choice A: Dizygotic, is incorrect. All dizygotic twins are dichorionic and diamniotic, as they are derived from 2 eggs and 2 sperm. This is the most common type of twinning overall.
Answer choice B: Monozygotic, split before day 4, is incorrect. Monozygotic twins who split before day 4 will develop separate chorions and separate amnions. This occurs in roughly one quarter of monozygotic twins.
Answer choice D: Monozygotic, split between days 8 and 12, is incorrect. Monozygotic twins who split after the blastocyst stage (day 8) but before embryonic disc formation (day 12) are monochorionic and monoamniotic. This is a rare occurrence.
Answer choice E: Monozygotic, split after day 12, is incorrect. Monozygotic twins who split after embryonic disc formation (day 12) are monochorionic, monoamniotic, and conjoined. This is a rare occurrence.
Key Learning Point
In monozygotic twinning, the number of amnions and chorions depends on when splitting occurs, with key points being the development of the morula/placenta (day 4), blastocyst/amnion (day 8), and embryonic disc (12).
