A single baby is conceived when a man's sperm fertilizes a woman's egg. The resulting embryo attaches itself to the woman's uterine wall and develops into a fetus. There are two types of twins: dizygotic and monozygotic. In dizygotic - - also called fraternal - - twins, the woman releases two eggs simultaneously and each egg is fertilized by separate sperms. This causes two separate embryos to form, each with its own amniotic sac and placenta. On the other hand, monozygotic - - also called identical - - twins are formed when the original fertilized egg splits into two during the first few days after conception. In this case, various scenarios occur; some monozygotic twins share amniotic sacs and placentas, and some don't.
Scientists say that fraternal or dizygotic twins occur when a woman hyperovulates, meaning she has a tendency to release more than one egg per ovulation. This can be an inherited trait, and a woman who is a fraternal twin is more likely to produce fraternal twins herself. Hyperovulation also can be caused by an increase in the follicle-stimulating hormone that is naturally present in older and heavier women. In addition, increased use of infertility treatments today is causing more twin births. When a woman undergoes in vitro fertilization, the doctor usually places several fertilized eggs into her uterus in case the first doesn't implant right away, often resulting in twins or other multiple births.
In rare circumstances, if a woman releases the two eggs several weeks apart and they are fertilized at different times, the two births can occur weeks apart as well. The twins can have different birthdays; this type of twinning is called superfetation. Furthermore, if the same woman had multiple partners, the two eggs could potentially be fertilized from two different people, causing superfecundation twinning.
Conjoined twins form when a fertilized egg gets split later in the process, causing an incomplete separation. Instead of producing monozygotic twins, the twins are born physically attached. Conjoined twins are classified according to the body part that attaches them, such as the torso, head, rump or arm. Often these twins are stillborn or live only a day or two. If the conjoined twins survive, they are surgically separated, if possible, soon after birth. There are only a number of nonseparated conjoined twins alive today.
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