Finally, the zygote may split extremely late, resulting in conjoined twins. Popularly known as Siamese twins, conjoined twins occur in 1 in 100,000 births or 1 in 200 MZ twins. Conjoined twins can be identified via ultrasound in the early part of pregnancy. Apart from sharing placentas and membranes, conjoined twins also can also share body parts and organs. Occurrence of conjoined twins happens when the zygote divides late and doesn't split totally. The twins can be linked at any part of the body but both will be joined at the same place.
Semi Identical Twins;
The 'semi-identical' twins are the result of two sperm cells fusing with a single egg, before becoming two embryos. An embryo created this way doesn't usually survive, but a few cases are known. Most of the time it goes totally unnoticed, and only shows up either if the two eggs have different genetic genders and the baby ends up with a mix of male and female gonads, or if it happens to be picked up on a genetic test that's done for other reasons (ex. paternity tests).
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