Question: My niece is expecting twins. Is it possible to know whether they're paternal or fraternal before they're born?
Answer: The answer to your question is "maybe."
Before I explain, I'd like to clarify the terms "paternal" and "fraternal." In the past, people used these terms to describe whether twins were from one egg or two, but the use of the term "paternal" is somewhat confusing, since all pregnancies are of paternal origin. So to clarify, we now use the terms "identical" and "fraternal"; identical twins come from one fertilized egg and fraternal from two.
One of the major advances in distinguishing between the two types of twins has been the development of ultrasound. However, when a twin pregnancy is evaluated by ultrasound, it's impossible to tell directly whether the twins are identical or fraternal.
That determination must be inferred by looking at two things — the sex of the fetuses and the number of placentas and sacs present. If one's a boy and one's a girl, they have to be fraternal. If the fetal sexes cannot be determined, or if they are the same sex, we can only guess which kind of twins are present from the appearance of the placenta(s) and sac(s).
Fraternal twins always have completely separate placentas, and therefore, separate sacs. But identical twins, or twins who come from one egg, can have one of three scenarios — two placentas/two sacs, one placenta/two sacs or one placenta/one sac.
When the egg divides relatively early in embryonic life, the twins will have their own placentas and sacs, just like fraternal twins. Therefore, it's impossible to tell the difference between fraternal twins and identical twins who have separate placentas unless their sexes are different. When the egg divides a little later in embryogenesis, the twins may share the same placenta or even the same sac. If there is one placenta or sac, then, by definition, the twins are identical.
It's helpful for the doctor to know ahead of time the kind of twin pregnancy a woman is carrying because different kinds of twins face different potential problems. For example, identical twins that share the same placenta may also share some of the same blood vessels in the placenta. That is, an artery or vein from one baby"s placental circulation may connect directly with an artery or vein of its twin, even if they each have their own sac. This can allow one twin to "steal" blood from the other, causing it to grow extra large at the expense of its smaller twin.
This phenomenon, called twin-twin transfusion, is relatively common and can be a cause for great concern because a large disparity in size can result in significant cardiovascular problems or stillbirth for the fetuses. I once saw a case of twin-twin transfusion where one baby weighed 2,000 grams and the other 1,000 grams. These "identical' twins were anything but identical.
Another dangerous twin situation is that of twins who share the same sac. Fortunately, these kinds of twins, known as monoamniotic twins, comprise only one percent of all twin pregnancies. Two babies in one sac is fraught with the possibility that one of them will wrap around the other's umbilical cord, causing the death of one or both. Conjoined, or Siamese, twins are a type of monoamniotic twins whose bodies never completely separate from each other in the twinning process.
In the United States, 1-2 percent of all pregnancies are twins, and about two-thirds of all twin pregnancies are fraternal. The identical twinning rate is relatively constant around the world, but the fraternal twinning rate is quite variable.
The phenomenon of fraternal twinning, which is a result of two eggs ovulating in one cycle, appears to be related to the levels of gonadotropins manufactured in a woman's pituitary gland — something that is affected by race, family genetics, age, and environment. For example, fraternal twins are much more frequent in Nigeria, whereas, they are less common in Japan. They are also more common as women approach age 35 and are less common during times of famine. Fraternal twins may run in some families, but identical twins do not show family clustering.
Stephen Lamb practices OB/GYN at the Millcreek Women's Center in Salt Lake City. He is also the co-author of "Between Husband and Wife." E-mail: slamb@desnews.com