A Texas woman got a quadruple Valentine's Day gift this year, giving birth to four babies -- two sets of identical twins. The twins were not the result of fertility treatments, the hospital said. Each pair of twins shared a placenta, the hospital said. Identical twins result when a fertilized egg splits into two embryos. Twins occur in about 2% of all pregnancies. Of those, 30% are identical twins. The odds of having two sets of twins at once is about 1 in 70 million, said Dr. Alan Penzias, associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School.