A 31-year-old primigravid woman undergoes routine ultrasonography at 12 weeks' gestation. The examination demonstrates a twin pregnancy in which the fetuses share a single placenta but are separated by a thin intertwin membrane. Both fetuses appear structurally normal and have appropriate growth for gestational age. The physician explains that these findings are the result of delayed division of a single fertilized ovum.
Division of the embryo most likely occurred during which of the following time periods after fertilization?
The correct answer is:
B) Days 4–8 (blastocyst stage before amnion formation)
Monozygotic twins arise from division of a single fertilized ovum. The timing of embryonic splitting determines the fetal membranes. Division during days 4–8, after formation of the trophoblast but before development of the amniotic cavity, produces monochorionic, diamniotic twins. These twins share a placenta (single chorion) but have separate amniotic sacs, making this the most common type of monozygotic twin pregnancy.
Answer choice A: Days 1–3 (morula stage), is incorrect. Division before differentiation of the trophoblast results in dichorionic, diamniotic twins, each with its own placenta (although the placentas may appear fused) and its own amniotic sac.
Answer choice C: Days 8–13 (after amnion formation), is incorrect. Division after formation of the amniotic cavity results in monochorionic, monoamniotic twins, who share both the placenta and the amniotic sac. These pregnancies carry increased risks such as umbilical cord entanglement.
Answer choice D: After day 13, is incorrect. Division this late is often incomplete, resulting in conjoined twins due to incomplete separation of the embryonic disc.
Answer choice E: Before fertilization, is incorrect. Dizygotic twins arise from fertilization of two separate oocytes by two different spermatozoa. Monozygotic twins always result from division of a single zygote after fertilization.
Key Learning Point
The timing of monozygotic twinning determines chorionicity and amnionicity:
Days 1–3: Dichorionic, diamniotic
Days 4–8: Monochorionic, diamniotic (most common)
Days 8–13: Monochorionic, monoamniotic
After day 13: Conjoined twins
