How Are Twins Passed Down?

Do you or a woman you know have twins? If so, you are probably familiar with people’s fascination by this fact. Actually, the subject of twins interests many people around the world. They wonder whether twins are a genetic trait, the probability of getting twins and any factors involved. They may ask "how are twins passed down?" The genetic assumption is not surprising considering the fact that some women who give birth to twins have other twins in their families. Then is it right?


Are Twins Genetic?

Yes and no! Genetically, there are two types of twins: identical and fraternal. Identical twins don’t result from a genetic trait, while fraternal twins are likely to run within families.

Identical (monozygotic) twins come from one egg that splits after fertilization. Identical twins are identical or near identical in many ways: same sex, similar physical appearance, etc. You are unlikely to tell identical twins apart unless you spend a lot of time with them. It is still not known why a fertilized egg splits to produce two embryos, but scientists think this is a random occurrence. This means that a mother to one set of identical twins is not more likely to get another set of identical twins than any other woman.

Fraternal (dizygotic) twins are simply siblings born from one pregnancy. They only look alike as far as they are siblings and may even be of different sex. Fraternal twins come from two eggs released at the same time and fertilized individually by different sperms. Women normally release a single egg in every cycle; however, probably due to their genetic makeup, some women may release two or more eggs which if fertilized result in twins or other multiple births.

How Are Twins Passed Down?

Generally, hyper-ovulation – release of more than one ovum in one cycle – is a genetic trait that can be inherited by a child from the mother. However, hyper-ovulation may happen due to other reasons. It has been established that a woman in her 30s is more likely to hyper-ovulate than a woman in her early 20s. It has also been established that getting twins does not skip generations. This may, however, appear to be the case when the twinning gene is inherited by a male who passes it to his daughter. It is worth to note that a man who inherits the twinning gene does not have an increased chance of fathering twins because his genetics have no effect on his female partner’s ovulation. A woman whose mother or grandmother has had fraternal twins is likely to get twins herself because she might have inherited the hyper-ovulation genes. 

What Affects the Chances of Having Twins?

Here are several factors that may play a part. 

1.     Previous Pregnancies

Research indicates that if you have had one set of fraternal twins, you are five times more likely than the average woman to have twins. Maybe you have the hyper-ovulation genes mentioned in the "how are twins passed down" part. It has also been found out that a woman who has had four or more pregnancies has a higher chance of getting twins.

2.     Race

Racial background has a bearing on the likelihood of getting twins. Nigerians have a very high twinning rate at five percent. Some studies suggest that this happens due to Nigerian’s high consumption of yams. You could try eating lots of yams if you are keen on getting twins. It has also been found that twinning is more common in African Americans and white Americans than in Asians and Hispanics.

3.     Obese or Tall Body

A study report shows that women with a body mass index (BMI) of 30+ and those, whose height is in the top 25th percentile, had a higher chance of getting fraternal twins.

4.     Maternal Age

If you are aged 35 or more, you have a higher chance of getting twins. This is because at this age, a woman’s body produces more follicle stimulating hormone (FSH) than at a lower age. Production of more FSH means that older women will not easily get pregnant; however, they have a higher chance of getting twins or multiples if they get pregnant.

5.     Fertility Treatments

Fertility treatments such as in-vitro-fertilization (IVF) are believed to increase the chance of a woman to have twins or multiple births by 20 to 40 percent. The increase is dependent on the number of embryos placed inside the uterus. There have been claims that IVF may also increase the chance of getting identical twins. These claims have, however, not been backed up by any studies. Besides IVF, fertility medications can increase your chances of getting twins or multiple births by between 10 and 30 percent because they act by stimulating the ovaries to produce ova.

6.     Diet

Studies show that a woman’s diet can play a part. If you eat lots of fatty foods, you increase your chances of getting twins over women who eat low fat or non-fat foods. The reason for this is likely because fat is required in supporting pregnancy. It is also possible that full fat foods affect a woman’s hormonal balance. In the case of Nigerian women, it has been reasoned that the yam staple could be the contributing factor in the increased chances of twinning and multiple births. Analysis of true yams shows that constituents of these tubers include a phytoestrogen that raises the amounts of FSH. This is thought to increase the chances of hyper-ovulation.

How Will I Know If I’m Having Twins?

After the answer to "how are twins passed down" you are probably wondering how you would know you are having twins.

When pregnant with twins, many women say the pregnancy indicators show earlier and there is an increased level of morning sickness. Pregnancy with twins may also mean that a woman has higher levels of the human chorionic gonadotropin (HCG) hormone, which is produced in a woman’s body during pregnancy. However, the sure way to find out if you are pregnant with twins is to go for an ultrasound test which can show the presence of twins in the womb by 8 to 14 weeks of gestation.