Slightly more than half of all twins born are male.
In conjoined or Siamese twins, the picture is complicated both by their extreme rarity and by their very low overall survival rate, which is variously given at 5 percent to 25 percent.
Conjoined twins are always identical, because they always come from a single fertilized egg and so are always the same sex. The developing embryo splits within two weeks of conception, but is only partly separated and continues to mature into two infants that are connected to a greater or lesser degree.
Many die at birth and many have severe birth defects. About 35 percent survive just one day. However, historical studies of about 600 sets of conjoined twins over five centuries list more than 70 percent of the surviving pairs as female.
In the United States there has been a sharp rise in multiple births overall, associated with the two overlapping and related trends older age at childbearing and increased used of fertility-enhancing therapies, many of which have multiple pregnancies as a side effect.
