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 ...

Twin fetuses endanger each other's life within the womb. July 29, 2008— -- After two and a half years of marriage, Kim and Nate Stroh of Humble, Tex., got the good news on New Year's Day that Kim ...

New reproductive technologies have yielded a vast array of novel variations on the two conventional twin types—identical and fraternal—as well as some curious twin-like sibships. When a Canadian ...