Miscarriage, also known as spontaneous abortion, is unfortunately quite common, occurring in about 10-20% of pregnancies. Many couples who experience a miscarriage go on to have healthy pregnancies afterward. Some research has suggested that a woman may be more likely to conceive twins after a miscarriage, but the evidence is mixed. In this article, we’ll review what the research says about whether miscarriage increases the chances of twins.
What causes twins?
Twins occur when two eggs are fertilized at the same time (fraternal or dizygotic twins) or when one fertilized egg splits into two embryos (identical or monozygotic twins). The chances of twins are influenced by a few key factors:
Age
The older a woman is, the more likely she is to release multiple eggs during ovulation. Twins rates steadily increase with maternal age up until about age 38, then decline again.
Family history
Having twins runs in families. If you have twins in your family, you are more likely to conceive twins. The father’s family history also matters.
Ethnicity
African Americans are more likely to have twins than Caucasians. Asian and Hispanic women have lower rates of twins. The reasons are not totally clear but may relate to differences in diet, body size, or genetic factors across ethnicities.
Prior pregnancy history
Having given birth to twins already makes it more likely to have them in future pregnancies. Some research also suggests women who have been pregnant before are more likely to ovulate multiple eggs.
Does miscarriage increase the chance of twins?
Some studies have found an association between miscarriage and subsequent twinning, leading to the theory that some factor related to the miscarriage makes conceiving twins more likely. However, the evidence is mixed, with other studies finding no link between miscarriage and twins.
Here is a summary of some key studies on miscarriage and twins:
Studies finding increased twinning after miscarriage
- A 2004 study of over 800 women in Scotland found that those who had 2 or more miscarriages were 2.4 times more likely to conceive twins afterwards compared to those with no miscarriages.
- A study of over 2,500 American women found that those with a history of miscarriage had 1.7 times higher odds of twins in their next pregnancy.
- An analysis of national birth data from 1997-2000 found women with a history of miscarriage had a 41% higher chance of twins compared to those with no history.
Studies finding no link between miscarriage and twins
- A study of over 22,000 Finnish mothers found no association between prior miscarriage and twins in the next pregnancy.
- A study of over 600 women in Israel found similar twinning rates in women with and without prior miscarriage.
- A study of 165,000 American women found no increased twinning risk after one or more miscarriages compared to no prior miscarriage.
Overall the evidence remains inconclusive, with some but not all studies finding miscarriage linked to higher twin rates. Larger, high-quality studies tend to show little to no effect.
Why might miscarriage be linked to twins?
There are a few theories about why miscarriage could potentially increase the odds of twins:
Higher FSH levels
After a miscarriage, levels of follicle stimulating hormone (FSH) remain elevated for a few months. FSH stimulates the ovaries to mature multiple follicles, which can result in multiple eggs being released at ovulation. However, research is mixed on whether FSH levels actually remain elevated after miscarriage.
Rapid return to fertility
Some research shows women are more fertile in the first few months after a miscarriage. The rapid return to ovulation combined with elevated FSH may favor the release of multiple eggs. However, other studies find no change in fertility after miscarriage.
Genetic or environmental factors
Shared genetic, dietary, or environmental factors could potentially contribute to both miscarriage and hyperovulation (release of multiple eggs). However, the specific factors have not been identified.
Chance/bias
Some statisticians argue there is no cause and effect. The association may be seen simply because women who conceive twins are inherently more likely to miscarry. Or bias and errors in data collection may produce an artificial link.
More research is needed to provide definitive evidence whether miscarriage directly leads to twins or if other factors are involved.
Other risk factors for twins
While the impact of miscarriage is unclear, other factors are well-established to increase the chances of twins:
Family history
Having a twin pregnancy already, or twins in your family makes you more prone to conceive twins. Genes likely influence the likelihood of hyperovulation.
Maternal age
As a women gets older, her chances of releasing multiple eggs goes up. Rates of fraternal twins rise steadily between ages 20 and 38.
Fertility treatment
Drugs used for ovarian stimulation and IVF can cause hyperovulation, leading to very high twin rates (up to 30%) with these treatments.
Birth order
Having given birth three or more times before is linked to increased odds of twins. The reasons are not entirely clear.
Race/ethnicity
As noted above, African Americans have the highest rates of fraternal twins, for reasons likely tied to diet, body composition, or genetics.
Risks of twin pregnancy after miscarriage
For women who do conceive twins after miscarriage, it’s important to be aware of potential risks:
Preterm birth
Over 50% of twin pregnancies result in preterm delivery, compared to 10% with singletons. Early birth poses many health risks.
Low birth weight
Twins are often born small due to crowded conditions in the womb and premature delivery. Low birth weight babies have higher risks of complications.
Preeclampsia
Preeclampsia, or high blood pressure in pregnancy, occurs 2-3 times more often with twins. It can be dangerous for both mother and babies if severe.
Gestational diabetes
Mothers carrying twins have 2-3 fold higher rates of gestational diabetes. Careful blood sugar control is vital to minimize risks.
Placental issues
separate placentas for each twin raises the risk of uneven blood flow, growth restriction, or cord accidents.
Working closely with your doctor for careful monitoring can help minimize risks and support the best possible outcome.
Conclusion
The evidence for whether miscarriage increases the chances of conceiving twins is mixed and inconclusive. Some studies have found women have higher twin rates after one or more miscarriages compared to those with no prior losses. However, other research has found no effect of miscarriage on the odds of twins.
There are some theoretical reasons why miscarriage could promote hyperovulation or twin conception, including elevated FSH, rapid return to fertility or shared genetic factors that predispose to both outcomes. However, the association remains unproven and disputed.
Regardless of miscarriage history, factors like maternal age, family history, and use of fertility treatments are well-established to increase twinning rates. Women who conceive twins after miscarriage should be aware of the higher risks these pregnancies can carry. More research is needed to clarify if miscarriage directly leads to twins or if the relationship is due to chance or other common contributing factors.
