Fraternal twins are the most common type of twins, where nearly 75 percent of twins are fraternal. As opposed to identical twins where one egg is fertilized with one sperm and splits into multiple embryos, fraternal twins are actually two separate eggs fertilized by two separate sperm, in a process known as dizygotic. This happens due to a number of contributing factors such as age, fertility treatments and heredity.
Getting Pregnant With Fraternal Twins
There are two ways to get pregnant with fraternal twins; through superfecundation or superfetation. Superfecundation is the more common way for women to naturally conceive fraternal twins. This is the method in which a woman releases more than one egg during ovulation, also known as hyperovulation, whether naturally or with the aid of fertility medications. If two eggs are fertilized by two sperm, a fraternal twin pregnancy will occur. Superfetation happens when a woman achieves pregnancy and later ovulates a second time which also results in pregnancy. With this method, the embryos will have different conception dates but will be born at the same time.
While 75 percent of twins are fraternal, the occurrence of twins varies largely based on geographic location and background. For instance, fraternal twins are most common in Africa, with rates as high as one in ever 20 pregnancies being a twin pregnancy. Asia holds the spot for least common fraternal twins, at one in every 500 pregnancies. Caucasians are somewhere in the middle with it occurring about one in every 125 pregnancies. Females who are a fraternal twin themselves, have a one in 17 chance of conceiving another set of fraternal twins. If you have a set of fraternal twins, you are twice as likely to conceive fraternal twins with a future pregnancy.
Similarities Between Fraternal Twins
Because fraternal twins are from two separate sperm, each comes with its own set of genes to attribute to physical characteristics and personality traits. Aside from sharing the womb and having the same birthday, fraternal twins are no more alike than any other sibling. Oftentimes they will look similar, as many siblings do, but not as closely as identical twins in most cases. However, because of their closeness from conception, fraternal twins are often as close to each other emotionally as identical twins.
A variety of factors contribute to conceiving fraternal twins. Hyperovulation, or the release of more than one egg during ovulation, is hereditary; therefore, if fraternal twins run in your family, you have a higher probability of conceiving fraternal twins. Due to hormonal changes, older women also have a higher chance of conceiving twins. Those who undergo fertility treatments such as In-Vitro Fertilization (IVF) have a 30 to 50 percent chance of conceiving fraternal twins as more than one egg is fertilized and implanted into the uterus.