Metformin has become a popular medication for the treatment of women with PCOS and insulin resistance . Numerous studies have found that some women who don't ovulate due to PCOS or insulin resistance, may start to have ovulation when using metformin. Although metformin is far less efficient for producing pregnancy and live births compared to clomid , a popular fertility medication, metformin continue to be prescribed by doctors commonly.

A persistent question about the use of metformin is when it should be stopped. A general principle of obstetrics is to not use a medication during pregnancy unless the medication has been well studied and the benefits outweigh the risks to the fetus. Since metformin is primarily used to treat diabetes (high blood sugar), some have suggested that it may be used to treat diabetes in pregnancy (gestational diabetes). Women with PCOS or insulin resistance develop gestational diabetes more commonly. Traditionally, gestational diabetes has been treated with diet and when necessary with a medication called insulin. Insulin is given as an injection up to 3 or 4 times per day. If a once day oral medication was safe and effective, then it would we a welcome change for women with gestational diabetes.

In 2009, doctors in the U. K. studied 200 women with gestational diabetes. 100 women were treated with insulin and 100 were treated with metformin. They found that women treated with insulin had greater weight gain during pregnancy than those treated with metformin. There was no difference in how often women in the two groups need to have labor induced or in the rate of cesarean section.

However, the incidence of pre-eclampsia was over four times higher in the metformin group (9% versus 2%). Statistical analysis showed that there was a six percent chance that these results arose purely by chance rather than an adverse effect of the metformin. Typically, in medical studies, an upper limit of 5% is considered acceptable. As a result, the authors of the study concluded that there was "no significant difference" in the risk for preecalmpsia. However, this is not an appropriate conclusion. It simply means that 200 women was not a sufficient number to answer the question.

Some previous studies have also found a higher incidence of blood pressure problems in women taking metformin during pregnancy. A 2000 study from the Netherlands also found an increase risk.

Luckily, no fetuses were lost in either treatment group. There did seem to be some benefits found in the metformin group. The risk of prematurity, neonatal jaundice and admissions to neonatal intensive care unit were lower in the metformin group.

Metformin should be continued to be studied in pregnant women. There is insufficient data at this time to recommend the routine use of metformin during pregnancy. Metformin should only be used in pregnancy under the supervision of a study protocol.