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Fraternal twins can be the same or different sexes.
Having a baby is a special time in a parent's life, and having twins can be especially exciting. In the case of fraternal twins, two separate eggs are fertilized and develop. Although the development of each fetus in the twin pair is essentially the same as for a single fetus, some special adaptations are needed for the mother's womb to support two babies through fetal development.
According to a 2008 review published in "Human Reproduction Update," one in 30 people in the U.S. or Europe is a twin. However, these statistics vary in other parts of the world, from 1 in 70 in Asia to 1 in 12 in parts of Africa. About two-thirds of twins are fraternal twins, technically called dizygotic twins, because there are two separate fertilized eggs. Unlike identical twins, who are exactly alike genetically, fraternal twins have the same degree of genetic similarity as any other siblings. Because they develop from separate eggs, fraternal twins can be different sexes or the same sex. The mother's genetic makeup is probably partly responsible for fraternal twins, as they tend to run in maternal families. A woman's likelihood of having fraternal twins also increases if she is over 35 and is about 5 times more likely to repeat in a later pregnancy once a first set of fraternal twins has been born. High levels of a pituitary hormone called FSH also predisposes a woman to having fraternal twins, although the reason for this is not well understood. Certain fertility treatments increase the likelihood that a woman will ovulate 2 or more eggs in a given month, raising her chances of having fraternal twins.
When a fetus implants in its mother's womb, one of the early and critical events in its development is growth of the placenta, which is partly formed by the fetus and partly by the mother's uterus. In early fetal development of fraternal twins, each embryo begins to form a placenta independently of the other. However, sometimes one or more parts of the placenta from each embryo fuse with each other, depending on how close the two implantation sites are to each other. If both implant on the same side of the uterus or near its upper region, the placentas may fuse partially as they enlarge. Unlike identical twins, however, fraternal twins always have separate membranes surrounding each fetus and separate umbilical cords. During the first trimester, essentially identical developmental events occur in both fetuses. Each fetus begins to take on the human body shape, developing recognizable arms and legs, and basic organ systems start to form. By the end of 9 weeks, each fetus in the fraternal pair is usually about 1 inch in length.
From about week 9 through birth, fetal development in fraternal twins involves final development of all the organ systems in preparation for birth. If one of the babies is a male, he will develop a recognizable penis by about week 12 to 14. The kidneys begin functioning between weeks 9 and 12, producing urine that becomes part of the amniotic fluid. Starting in week 13, both fetuses grow rapidly and begin to deposit calcium in their bones, allowing a doctor to see their skeletons on x-ray by week 16. After week 16, the twins continue to grow -- but at a slower pace. Each baby gains weight and some subcutaneous fat during this time. Although their growth rate is generally similar to that of a single baby, each twin may weigh slightly less at birth than a single, nontwin baby. Occasionally, one twin may be larger than its sibling at birth, a situation called discordance. This could be due to genetic differences between the babies or subtle differences in blood supply caused by the location of each placenta in the womb.
Chimerism and Other Issues
During early development of the separate placentas of fraternal twins, blood vessels from each placenta may join and form a link, called an anastamosis. Although this generally has no negative effect on future fetal development, it can lead to a situation called blood chimerism, in which each twin has red blood cells from the other twin after birth. Usually detected when the twins have a blood test after birth, blood chimerism develops in about 8 percent of fraternal twin pairs and has no impact on the baby's future development. Because fraternal twins are genetically different, their likelihood of developing a genetically-based problem is the same as for any other pair of siblings. However, because they share the mother's uterus, each is equally likely to be affected by any potentially damaging environmental factor, such as alcohol consumed by the mother. If you have a family history of fraternal twins and are pregnant or planning to become pregnant, discuss prenatal care and other issues related to having twins with your family doctor or obstetrician, who can advise you on the best course of care for your situation.