Twins can arise when two distinct eggs fertilize in the womb or when a single fertilized egg divides into two embryos. Having twins is more prevalent today than it was previously. Twin births have nearly doubled in the previous 40 years, according to the Centers for Disease Control and Prevention (CDC). The exact cause of twinning is not known, but it is thought to be related to genetics, age, weight, fertility drugs, radiation therapy, alcohol consumption, smoking, stress, and other factors. Twins may also be born conjoined, where they share organs such as hearts, lungs, stomachs, intestines, kidneys, livers, legs, arms, and chests. In these cases, one body cavity may be partially blocked by the presence of the other twin. Or perhaps both babies may survive but one will not develop normally. When twins are born at different times, they are called discordant twins. When one twin is stillborn, disconcordance is evident. When both twins die shortly after birth, concordance is evident. Discordant twins are more common than concordant ones. After all, not every pair of eggs will become identical twins, just like not every pair of sperm will result in a pregnancy. The chance of becoming pregnant is about 50 percent if only one ovary is involved and 25-30 percent if both are used.