Metformin in Pregnancy: What It Means for Your Baby, From the First Days to the First Year of School

A family guide to three studies — Brinkmann et al. (2026), Ryssdal et al. (2025) and Gordon et al. (2026) — on newborn blood sugar, childhood allergy and development after metformin before birth

Many pregnant women take metformin, a blood-sugar tablet that also reaches the baby. Three new studies suggest that, compared with insulin injections, it slightly lowers the chance of low blood sugar after birth and does not appear to affect children's development when they start school, while one small study in mothers with polycystic ovary syndrome raises an unconfirmed question about allergy and eczema.

If you took metformin while pregnant, it is natural to wonder whether it affected your baby. This article explains what researchers have found so far, what remains uncertain, and what it means for your family. Most of the news is reassuring.

Why doctors started using metformin in pregnancy, and why questions remained

Metformin is a tablet that has been used for many years to lower blood sugar in people with diabetes. In pregnancy it is mostly given for gestational diabetes, a type of diabetes that appears during pregnancy and usually goes away after birth. It is also used by some women with polycystic ovary syndrome (PCOS), a common hormonal condition that can affect periods and fertility, and by women with type 2 diabetes.

For a long time, insulin injections were the usual treatment when diet changes were not enough. A large study published in 2008, called the Metformin in Gestational diabetes (MiG) trial, with sites in Auckland, New Zealand, and Adelaide, Australia, compared metformin with insulin in 751 women [1][2]. Babies did about equally well with either treatment, and many more mothers said they would choose metformin again (about 77 in 100, compared with 27 in 100 for insulin) [1]. Metformin use in pregnancy has grown in some countries [3].

But there was always an important difference between the two medicines. Metformin passes through the placenta, the organ that connects mother and baby, so the baby receives levels close to the mother's [4][5]. Research did not find more birth defects in babies exposed early in pregnancy [6], but families and doctors wanted to know about effects that might show up later. Some studies found that metformin-exposed babies were a little lighter at birth but grew a little faster afterwards [7], and in mothers with PCOS, more exposed children were overweight at age 4 (about 32 in 100 compared with 18 in 100) [8]. A later review of many studies found that weight differences did not last as children got older [9]. Studies of children's thinking and learning were mostly reassuring, but they were small, and the combined results on developmental delay came only from babies and preschool children [10][11]. The three new studies in this article fill in some of these gaps.

Three studies, three different questions

The first study, by Brinkmann and colleagues in Denmark and Ireland, combined the results of earlier trials in which women with gestational diabetes were given either metformin or, in most trials, insulin [12]. In a trial, a fair process like a coin toss decides which treatment each woman receives, which makes comparisons trustworthy. The researchers combined ten trial reports (covering nine separate trials) and six follow-up studies of children and mothers. Women with PCOS were left out on purpose.

The second study, by Ryssdal and colleagues in Norway, looked again at two trials called PregMet and PregMet2 [13]. In those trials, carried out at hospitals in Norway, Sweden and Iceland, pregnant women with PCOS were given either metformin or a dummy tablet (a placebo) from early pregnancy until birth [14][15]. The researchers looked at infections in 634 mothers, and at allergies in 145 children from PregMet and an earlier, smaller trial, who were examined at about 8 years of age in Norway. Children from PregMet2 were not followed up. These questions were added long after the trials were designed.

The third study, by Gordon and colleagues in Victoria, Australia, used health records for 177,409 children born between 2009 and 2020 [16]. Of these, 1,095 had been exposed to metformin before birth. In their first year of school, at age 4 to 6, each child's teacher completed a national check called the Australian Early Development Census, which covers physical health, social skills, emotional maturity, language and communication.

What the studies found about newborn babies

A very large international study showed that the higher a mother's blood sugar in pregnancy, the more of a marker of the baby's own insulin is found in the umbilical cord blood, and the more likely the baby is to have low blood sugar after birth [17]. Low blood sugar in a newborn is called neonatal hypoglycaemia. This is why babies of mothers with diabetes usually have heel-prick blood sugar tests after birth.

Brinkmann's study found that babies whose mothers took metformin were less likely to have low blood sugar than babies whose mothers took insulin [12]. Out of every 100 babies, about 15 had low blood sugar with insulin compared with about 10 with metformin. Put another way, for every 22 mothers treated with metformin instead of insulin, one baby avoided low blood sugar. Babies were also about 69 grams lighter on average, roughly the weight of an egg. It is also reassuring that when low blood sugar is found and treated, most babies do well: a study in New Zealand found that babies whose low blood sugar was treated did not have more problems with development at age 2 [18].

A new question about allergy and eczema

Ryssdal's study found something unexpected. Mothers with PCOS who took metformin had fewer infections, such as colds and stomach bugs, during pregnancy: about 42 in 100, compared with 52 in 100 who took the placebo [13]. But children whose mothers took metformin in pregnancy more often had allergies and eczema, a skin condition that causes dry, itchy patches. Of 74 children exposed to metformin, 13 were counted as having an allergy, compared with 3 of 71 children who were not. Eczema affected 26 of 74 exposed children compared with 13 of 71. Asthma did not differ. The researchers had earlier found that metformin changed some immune signals in the mothers' blood [19], which could offer a possible explanation.

This finding needs careful handling. The study was small, only about half of the children invited took part, and "allergy" was judged mainly from whether parents reported giving the child allergy medicine (antihistamines), not from allergy tests. The trials were not designed to look at allergy, and the authors know of no other study that has looked at allergy after metformin before birth. The authors themselves say more research is needed and urge caution. It also applies to mothers with PCOS who started metformin early in pregnancy, not to the more common situation of gestational diabetes treated later in pregnancy.

What the studies found about development at school

Gordon's study looked at the question many parents care about most: is my child ready for school? At first, children exposed to metformin seemed slightly more likely to be "developmentally vulnerable", meaning they scored in the lowest tenth in at least two of the five areas: about 18 in 100 compared with 14 in 100 [16]. But the mothers who took metformin were different in other ways. On average they had a higher body weight, and they were far more likely to have diabetes or PCOS. These health conditions, rather than the medicine, may affect a child's development.

When the researchers carefully accounted for these differences, the gap disappeared: exposed children were no more likely to be developmentally vulnerable than other children. The same was true when they looked only at mothers with diabetes, and only at children exposed in early pregnancy. A New Zealand study had earlier found no meaningful difference in children's behaviour at a health check before starting school after metformin compared with insulin [20]. A study like this cannot prove that metformin is completely safe, but it found no sign of harm.

What this means for you and your baby

If you took metformin for gestational diabetes, the overall message is reassuring. Compared with insulin, it slightly lowers the chance of low blood sugar after birth, and a very large study found no link between metformin and development at school entry. Your baby will still have the usual blood-sugar checks after birth, because the mother's diabetes, not the medicine, is the main reason for them.

If you took metformin for PCOS from early pregnancy, the allergy finding is worth knowing about but is not a reason for worry or for special tests. Mention it if your child develops eczema or allergy symptoms, and follow the usual advice from your health visitor or doctor. The decision to use metformin in pregnancy is made with your obstetric or diabetes team, who can weigh your own situation, and medical guidelines in different countries differ on when metformin or insulin should be offered first [12].

What researchers are working on next

Researchers want to follow children exposed to metformin for longer, into their teenage years, to look at growth, heart and metabolic health, and learning. They also want to repeat the allergy findings in other groups of children, using proper allergy tests, and to learn whether the dose or the timing of metformin in pregnancy makes a difference.

Looking back on what we now know

Metformin reaches the baby, and families are right to ask what that means. The best current evidence suggests a small benefit for newborn blood sugar compared with insulin, no sign of harm to development at the start of school, and one unconfirmed question about allergy and eczema after early-pregnancy use for PCOS that researchers are now working to answer.

References

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