How Twins Are Conceived?

Twins happen when a woman’s body releases more than one egg in a single cycle, or when one fertilized egg splits into two. About 3% of births in the United States are twins. Most people know that twins run in families, but the biology behind how twins are conceived is more specific than many realize. The process depends on which type of twin you are talking about, and the two types are not created the same way.

How Are Fraternal Twins Conceived?

Fraternal twins, also called dizygotic twins, come from two separate eggs and two separate sperm. During a normal menstrual cycle, the ovary releases one egg. In a fraternal twin cycle, the ovaries release two eggs. Both eggs get fertilized by different sperm. Each fertilized egg implants in the uterus and develops as its own embryo.

Fraternal twins are no more genetically similar than any two siblings born at different times. They can be the same sex or different sexes. They each have their own placenta and their own amniotic sac in most cases.

The release of two eggs in one cycle is what makes fraternal twins happen. This can occur naturally, or it can be triggered by fertility medications. Some women naturally release more than one egg in a cycle more often than others, which is why fraternal twins can run in families.

How Are Identical Twins Conceived?

Identical twins, also called monozygotic twins, start as a single fertilized egg. That egg splits into two separate embryos within the first two weeks after fertilization. The two embryos share the same DNA, which is why they look alike and are always the same sex.

The cause of the split is not fully understood. It does not appear to be inherited. Identical twinning happens at roughly the same rate across all populations and does not run in families.

Identical twins may share a placenta or have separate placentas depending on when the split occurs. If the egg splits within the first few days, each twin may have its own placenta. If it splits later, they are more likely to share one. The timing of the split also affects whether they share an amniotic sac, which carries some additional pregnancy risks.

What Role Do Genetics Play in Having Twins?

The genetics of twinning only applies to fraternal twins. A woman who releases two eggs in a cycle is more likely to have fraternal twins, and this tendency can be passed down through the mother’s family. The father’s genetics do not influence whether a woman releases multiple eggs.

Hyperovulation is the term for releasing more than one egg in a single cycle. This trait appears to have a genetic component, but no single gene has been identified as the cause. Some research suggests that a gene involved in hormone regulation may play a role, but the evidence is not conclusive.

Age is a stronger factor than genetics. Women over 30 are more likely to release multiple eggs in a cycle. This is partly because hormone levels change with age. The rate of fraternal twins rises through the mid-to-late 30s and then declines as menopause approaches.

What Increases the Chance of Conceiving Twins?

Several factors are well established as increasing the odds of a twin pregnancy. None of these guarantee twins, but they shift the probability.

• Fertility treatment: In vitro fertilization (IVF) and ovulation-stimulating medications raise the chance of multiple eggs being released or transferred. This is the largest factor in rising twin rates over the past few decades.
• Maternal age: Women in their mid-30s have higher rates of fraternal twins than younger women.
• Family history: If the mother’s side has a history of fraternal twins, her odds are somewhat higher.
• Number of previous pregnancies: Women who have given birth before are slightly more likely to conceive twins.
• Height and body mass: Some studies suggest taller women and women with a higher body mass index have slightly higher rates of fraternal twins, but the reason is not clear.

Race also plays a role. Women of Nigerian descent have among the highest rates of fraternal twins in the world. Women of East Asian descent have among the lowest. The reasons for these differences are not fully understood.

Can You Increase Your Chances of Twins Naturally?

Internet sources often claim that certain foods, supplements, or timing of intercourse can increase the odds of twins. The evidence for most of these claims is weak or absent.

Folic acid is sometimes mentioned as a twin booster. One older study suggested a link between folic acid supplementation and higher twin rates, but later research has not confirmed this. Folic acid is still recommended during pregnancy for preventing neural tube defects, but you should not take extra folic acid expecting twins.

Yams and cassava are another popular claim. Some research in one region of Nigeria noted higher twin rates among women who ate a specific type of wild yam. No controlled study has confirmed that eating yams increases twin conception. The evidence is anecdotal at best.

Timing intercourse to coincide with ovulation does not increase twin rates. The idea that sperm survive longer and fertilize a second egg released later is not supported by research. Ovulation typically involves the release of one egg, and the timing of intercourse does not change how many eggs are released.

There is no proven natural method to increase your chances of having twins. If you are trying to conceive twins specifically, the most reliable path is fertility treatment, which carries its own risks and should be discussed with a doctor.

What Are the Risks of a Twin Pregnancy?

Twin pregnancies are considered higher risk than single pregnancies. This is not meant to alarm you, but it is important to understand the facts.

Preterm birth is the most common complication. Twins are often born before 37 weeks of gestation. The average twin pregnancy lasts about 35 weeks, compared to about 39 weeks for a single baby. Babies born early may need time in the neonatal intensive care unit.

Low birth weight is common in twins, even when they are born near full term. This is because two babies compete for space and nutrients in the uterus.

Preeclampsia, a condition involving high blood pressure during pregnancy, occurs more often in twin pregnancies than in single pregnancies. Gestational diabetes is also more common.

Identical twins who share a placenta face an additional risk called twin-to-twin transfusion syndrome. In this condition, blood flows unevenly between the two babies through shared blood vessels in the placenta. This condition requires specialized monitoring and treatment.

Most twin pregnancies result in healthy babies. But the higher risks mean that prenatal care is more intensive. Women carrying twins typically have more frequent ultrasound appointments and closer monitoring of blood pressure and blood sugar.