Here at InfertilityTV, I get a lot of questions from couples who are trying to have twins
about the chance for twins with fertility treatments. Much of what is out there is wrong
so today I will separate the fact from fiction
Most people are familiar with the fact that there are basically two types of twins. The
first type are identical twins. Identical twins arise from a split in the embryo shortly
after fertilization.
Here are some facts about identical twins
Identical twins are uncommon. Only, about 3-4 of every 1000 births are identical twins.
Identical twins make up about ⅓ of all twins With a few rare exceptions, this doesn't seem
to vary much based on where you live or other factors
Identical twins don’t generally run in families, though there are some very rare families where
it might happen. These rare families point to the possibility
that there may be genes that increase the chance for identical twins , though scientists
have not yet been able to pin down any specific genes yet.
The fertility medication Clomid is well known to increase the chance for fraternal twins
but this 2006 study also found that the proportion of identical twins was higher when twins occurred
with Clomid compared to a group of naturally occurring twins.
The only well documented factor that increases the risk of identical twins is IVF. Growing
embryos in the laboratory seem to increase the chance that an embryo will split. The
longer they stay in the lab, the greater the chance.
For embryo transfers that occur after growing embryos for 2-3 days, identical twins occur
about 1-2% of the time Waiting until the blastocyst stage at 5 to
6 days increases the chances to about 2-3% and making a hole in the shell surrounding the
embryo, known as "Assisted Hatching" roughly doubles the chance that the embryo will split
Fraternal twins are a lot more interesting. Fraternal twins arise from fertilization of
two different eggs by two different sperm. There are several factors that seem to influence
a woman having fraternal twins naturally, that is without the use of fertility drugs.
Geography Sub Saharan Africa sees about 23 fraternal
twins for every 1000 births. Asian countries have about 5 per 1000 births.
The US is in the middle with about 10 to 12 per 1000 births that occur naturally
Body type Tall women appeared to have one and a half
to two times the chance for twins in some studies but not in others
Women with a BMI over 30 had a 62% higher chance of twins and women with a BMI less
than 20 had a 38% lower chance for twins.
Genetics. Fraternal twins do run in families. For a given couple, if twins run in the female’s
family, or if the woman herself is a twin, the couple has a higher chance for twins themselves.
If twins run in the male’s family, the couple doesn’t have a higher chance for twins but
if the couple have daughters, they will have a higher chance for twins. The overall impact
of family history may increase the chance for a twin by 1 and a half to two times - so
about 15 to 20 twins per 1000 deliveries.
Age. It is true that as women get older, they have a higher chance of releasing more than one
egg at the time of ovulation and so - they have a higher chance of a twin pregnancy.
The impact here is pretty strong. Women in their late 30s and early 40s have about four
times the chance for twins compared to women in their late teens. The twin rate in older
women is about 40 per 1000 births
What about food? Yams - Some people have theorized that the
reason why some regions in Africa, like Nigeria, have a higher twin rate is because of the
presence of yams in their diet. The evidence here, however, is extremely weak.
Dairy products - One small study suggested that the milk from cows that received growth
hormone would have higher levels of a hormone called IGF-1 and that women who drank the
milk from these cows would therefore have higher IGF-1 levels themselves - which would in turn,
stimulate the ovaries to produce multiple eggs. However, studies in women who drank
a lot of milk found no increase in their IGF-1 levels. Dairy products are unlikely to influence
the chance for twins
What about medications? Metformin - Metformin is a medication that
is used to induce ovulation in women with PCOS. Metformin does not increase the risk
for twins. Folic acid - folic acid or folate is a supplement
that all women should take if they are planning to become pregnant since it reduces the risk
of certain birth defects. The evidence is mixed here - with some studies showing a slight
increase in twins and other studies finding no increase. At this point, the weight of
evidence argues against any effect. Letrozole - This medication is used to induce
ovulation in women with PCOS. Studies have found a twin rate of 10% or higher or at least
100 twins per 1000 pregnancies Clomid or clomiphene citrate has about the
same rate of twins as letrozole Injectable fertility medications like Follistim
or Gonal F has a twin rate of 25 to 30% and a pretty substantial rate of triplets and quadruplets
With IVF,. the rate of fraternal twins depends primarily on the number of embryos transferred.
If two embryos are transferred, in a world class IVF program like IVF1, the chance for
twin pregnancy exceeds 60%
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