Fetal Development of Fraternal Twins

How Fraternal Twins Occur

Women have two ovaries. During a normal menstrual cycle, the pituitary gland in a woman's brain releases follicle stimulating hormone (FSH). During a normal cycle, the FSH incites one ovary to develop an egg. However, some months the FSH incites both ovaries to develop an egg. When the pituitary gland releases luteinizing hormone (LH), which normally causes ovulation, both ovaries ovulate and an egg is released from each ovary. After sexual intercourse, the sperm travel and can fertilize both eggs, creating two zygotes. If both zygotes develop and attach to the uterine lining, the woman becomes pregnant with fraternal twins.

How Fraternal Twins Develop Similarities and Differences In Utero

Fraternal twins are not any more similar than two siblings that were born years apart to the same parents. Two individuals could have over one trillion babies without ever coming up with two that are exactly the same. These two babies can be the same or different in a number of ways, including blood type, gender, hair color or eye color. It is also worth noting that one can have a congenital defect or mutation, like Down's Syndrome, and the other could be perfectly normal. Their separate genes, which come from two different eggs and two different sperm, will determine all of this, for better or worse.

Fraternal Twin Gestation

After implantation, each fetus will develop its own placenta. It is possible, however, that these two placentas can merge into one during gestation. Each baby will also have its own umbilical cord. Non-genetic conditions, like fetal-alcohol syndrome, will likely effect each twin the same, as anything that can pass one placental barrier will likely pass the other. Birth weight with fraternal twins can vary as well and should not, on its own, be cause for concern. An obstetrician will give the mother an idea of acceptable weight ranges for her developing babies. With mixed-gender fraternal twins, there may be a wider discrepancy in fetal development, as boys traditionally develop certain things, like lungs, later than girls during gestation.

Resources

• Babyzone.com has forums and support networks for mothers who are expecting