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Two individuals are monochorionic monoamniotic identical twins. The twins were conceived naturally with no complications during the pregnancy.

How and when in development did the twinning process occur?

A

Both twins were formed from separate eggs and sperm that were both fertilized simultaneously and naturally implanted at the same time.

B

The twins resulted from one zygote that spontaneously split into two when the embryo was at the morula stage (days 1-3).

C

The twins resulted from one zygote that spontaneously split into two at the blastocyst stage but prior to implantation (days 4-8).

D

The twins resulted from one zygote that spontaneously split into two after implantation but before the formation of the embryonic disc (days 8-13).

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