Twins occur in about one out of 90 human pregnancies in the US. As a rule, at conception, one sperm fertilizes a single egg to produce a zygote. Two thirds of twins are fraternal, with fertilization ...
Two embryos split to create two sets of identical twin quadruplets. April 12, 2011— -- "Babies, babies, babies." That's how Miranda and Josh Crawford now describe a typical day after their two ...
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