It's vacation season. I've been off much of the past week as my brother is visiting with his family from New Brunswick. My brother has 12-year-old fraternal twins who stay up far too late, get up far too early, and have more energy than anyone should have in 30 degree heat while on vacation. I am barely keeping up. We have no family history of twins and neither does my brother's wife; she was just 30 when they were born and didn't use any fertility medications. So why did the couple get twins?

First some background... there are two types of twins: fraternal and identical. Fraternal twins result when the woman releases two eggs which are both fertilized by different sperm and two separate embryos are created. So fraternal twins are just like any other siblings, only born at the same time. However, identical twins result when one egg is fertilized by one sperm but then the embryo that is formed divides into two. Since they came from the same sperm and egg, they are genetically identical.

In North America, fertility treatments create about 1/3 of all twins and natural conceptions create 2/3. The chances you'll have fraternal twins on your own is 1 in 80 but with fertility treatment it's about 1 in 4. The chances you'll have identical twins on your own is about 4 in 1000 while with fertility treatment, it's slightly increased to 2%. You are especially more likely to have twins, specifically fraternal twins, if:

  • you have a family history of twins on your mother's side.
  • you are Japanese or Nigerian.
  • you are over age 35.
  • you are obese (BMI > 30 kg/m2).
  • you are tall (> 5'5").

Many of my patients want twins. They have been struggling for months or years to conceive and the idea of having two at once is very appealing. The problem is that twin pregnancies are complicated and the twins are at higher risk of handicap and other medical issues. Let's review the risks for fraternal twins, since the vast majority of twins are fraternal:

  • 60% chance of being born early or premature (before 37 weeks of pregnancy).
  • the risk of an anomaly is double since, after all, there are two babies. So if you have a 5% chance of having an abnormality, with two you now have a 10% chance.
  • the mother is more likely to develop gestational diabetes (which often needs to be treated with insulin injections) and high blood pressure.
  • over half of twins are born by cesarean section.
  • women lose twice the amount of blood at childbirth when giving birth two twins compared to a singleton pregnancy, increasing the risk of needing a blood transfusion.
  • twins are 7 times more likely than a singleton to have a major handicap such as cerebral palsy.
  • twins are 4 times more likely than a singleton to die in the first month of life.

This all sounds rather scary but most of us know healthy happy twins, like my brother's. Most twins do just fine, but as you can see from the list above, many do not. Since the goal of fertility treatment is to create healthy babies and minimize risk to the mother, we favor women having one baby at a time.

I don't know why my brother and his wife had twins as they have none of the risk factors but my sister-in-law would say she is an overachiever, even when it comes to ovulating -- she did it two eggs at a time!