Can you picture yourself going for your first ultrasound of pregnancy? Nervous anticipation and eagerness to see and hear that heartbeat. And then your sonographer tells you that everything looks great, only it's not one baby; you are having twins! What would your reaction be? Some may feel like they won the lottery, but many start to curse like a sailor. According to the Center for Disease Control (CDC), the probability of having fraternal, also know as, dizygotic twins has drastically increased from 1980-2009.
There are two theories as to why the rate of non-identical twins is on the rise. The natural theory suggests that as a result of people choosing to have children later in life there is a higher possibility for twin birth. That’s right, although it can be more difficult to get pregnant as you get older, there is a higher chance for fraternal twin birth in older women. As women reach their 30s and 40s ovulatory cycles become less regular and rather than releasing one egg per cycle they may release two or even three eggs per cycle. The release of multiple eggs allows for the possibility of multiple eggs being fertilized and then the surprise ultrasound.
The facilitated theory attributes the rise in fraternal twins to the increased use of of fertility treatments such as In Vitro Fertilization (IVF). IVF typically garners multiple emobryos, and more than one may be implanted to increase the chance of a pregnancy. But that also increases the chances of a twin pregnancy (or higher multiples).
There are other traits that play a factor, such as race, diet, and genetics. Women of West African descent are more likely to have twins than any other race, women of Chinese descent are the least likely. Mothers on low-protein, vegan diets are less likely to have twins as well.
Identical twins also known as monozygotic twins are less likely than fraternal (dizygotic) twins. Identical twins are the result of fertilization of one egg by a single sperm and the resulting fertilized egg then splits. This creates genetically identical babies who share about 100% of the genes. Whereas fraternal twins are more like siblings and share roughly 50% of their genes.
The type of identical twins depends on the number of days it takes the fertilized egg to split after fertilization. When the egg splits, there is a creation of different sacs. If the eggs splits 3-4 days after fertilization the babies will each have their own inner and outer sac (dichorionic/diamniotic). If the egg splits 3-8 days after fertilization the identical twins will share and outer sac but each have their own inner sac (monochorionic/diamniotic). When the division takes place between 8-13 days after fertilization, they share the inner and outer sac (monochorionic/monoamniotic) If the division occurs after day 13 of fertilization, they are in the same sacs and there is then a possibility for conjoined twins.