An egg becomes fertilized when a sperm penetrates it, creating a zygote. For the first eight weeks, the developing baby is called an embryo. It looks like a tadpole. At about three weeks ...

Because the zygote originated from the same egg and the ... and this affects one baby but not the other," says Carolina Bibbo, M.D., a maternal-fetal medicine specialist with a clinical focus ...

Half becomes your baby, while the other half forms the placenta, which is how she’ll eat and drink while snuggled in your uterus. Your little zygote has now officially turned into an embryo.