'Mounjaro babies': Weight-loss drug may undermine contraceptive pill, raise pregnancy risk

"I'd heard a lot about 'Mounjaro babies,' but I never thought it would happen to me."

The 36-year-old woman, identified only as A, had long assumed pregnancy was out of the question. She was severely obese — 103 kilograms at 165 centimeters — had polycystic ovary syndrome and had been on oral contraceptives for years because of it. She and her husband rarely saw each other, and she had in effect ruled out the possibility of conceiving.

Then, about three months after she started taking Mounjaro for health reasons, she learned she was five weeks pregnant. Her doctor suggested the drug may have made conception possible. "I was too complacent about being on the pill and never even considered the possibility of getting pregnant," she said in an interview. "The baby came without any preparation, but it happened under difficult circumstances, so I want to do my best to raise the child."

GLP-1-based obesity treatments such as Mounjaro and Wegovy are drawing growing attention for their potential effect on women's fertility. Last year, the UK's Medicines and Healthcare products Regulatory Agency (MHRA) urged women using GLP-1-based drugs to take extra care regarding pregnancy and contraception, after receiving more than 40 reports of unintended pregnancies among users.

Anecdotal accounts of unexpected pregnancies during GLP-1 treatment have multiplied abroad, giving rise to terms such as "Mounjaro baby," "Wegovy baby" and "Saxenda baby." As the use of obesity drugs including Wegovy and Mounjaro spreads rapidly in South Korea, similar stories have begun circulating on social media.

South Korean medical professionals also believe obesity drugs can affect a woman's chances of conceiving. Women who take oral contraceptives while on Mounjaro face particular risk. Tirzepatide, Mounjaro's active ingredient, slows the movement of food and drugs from the stomach to the small intestine, which can reduce absorption of oral contraceptives taken at the same time and diminish their effectiveness.

"Pharmacokinetic research has shown that when tirzepatide and oral contraceptives are taken together, the pill's maximum blood concentration — known as Cmax — can fall by as much as 66 percent," said Cho Byung-gu, administrative director of the Korean Association of Obstetricians and Gynecologists. "Women are advised to use an additional contraceptive method, such as condoms, or a non-oral method for four weeks after starting Mounjaro or increasing the dose."

A rise in oral contraceptive use among women of reproductive age is also seen as a factor amplifying the risk of unintended pregnancy. "Younger women today have less resistance to taking the pill than in the past and tend to want to manage their own contraception," one pharmacist said. "More women are also taking oral contraceptives not just for birth control but for conditions like polycystic ovary syndrome or acne."

Even women not on oral contraceptives may find their chances of conceiving increase once they start obesity medication. Weight loss and improved metabolism can restore ovulation that had previously been irregular. Women with polycystic ovary syndrome or obesity-related insulin resistance are particularly affected: elevated insulin levels stimulate the ovaries to produce androgens, disrupting ovulation. When GLP-1-based treatment reduces weight and improves insulin resistance, androgen production falls and normal ovulation can resume.

"A meta-analysis pooling existing clinical trials in women with polycystic ovary syndrome found that the spontaneous pregnancy rate among those who received GLP-1-based treatment was statistically significantly higher than in the control group," Cho said. "Women who previously had ovulation problems due to obesity or metabolic disorders need to pay closer attention to contraception, because their fertility can increase as their metabolism normalizes."

Obstetricians are already seeing patients they suspect became pregnant because of obesity medication. "Since around last year, the first thing I check when a patient comes in after a contraceptive failure is whether she has been using an obesity drug," another obstetrician said. "If she has, the drug is treated as a likely contributing factor."

There are also concerns that fetuses could be exposed to the drugs if a woman continues taking them without knowing she is pregnant. Animal studies have found that exposure to GLP-1-class drugs is associated with fetal abnormalities, growth delays and increased risk of embryo and fetal death. No clinical trials have been conducted in humans. The medical community says the risk cannot be ruled out given the lack of sufficient safety data for use during pregnancy.

Pharmacist B, who works at a pharmaceutical company, said the industry has moved beyond treating the link between obesity drugs and increased fertility as a mere hypothesis. "Research currently frames it as a 'hypothesis,' but describing it that way is itself quite a conservative stance," B said. "Within the industry, it is accepted as a given that obesity drugs can affect a woman's chances of getting pregnant."

"The product information for Mounjaro and other obesity drugs explicitly states that fetal safety during pregnancy has not been established," B added. "In plain terms, that means there is a possibility of harm, and the pharmaceutical company is not taking responsibility for that outcome. The phrase 'safety has not been established' should not be taken lightly as simply meaning 'we don't know yet.'"