A new study finds that getting pregnant after a woman gets her “tubes tied” – the colloquial expression for permanent female surgical sterilization – may be surprisingly common.

The study, published in the New England Journal of Medicine Evidence, examined survey data of more than 4,000 women who reported tubal ligations, the formal term for a range of surgeries that clamp or remove fallopian tubes. Researchers found that 3-5% of these women reported pregnancy after surgery.

“Tubal sterilization is an important form of contraception and for some people it is the right form of contraception,” said Dr Eleanor Bimla Schwarz, an internist at the University of California San Francisco and the lead researcher on the study.

“It’s just every single form fails at some point, so we have to know about alternatives that are effective. And, probably, we just need to make our peace that there will always be some need for abortion services, because even methods we think of as highly effective do fail and not that infrequently.”

The study is especially salient since the US supreme court overturned Roe v Wade in 2022. Dozens of states have since banned and severely restricted abortion access, and interest in permanent female surgical sterilization has increased among women aged 18-30, research has found.

Researchers conducted the study by analyzing data from four iterations of the US National Survey of Family Growth from 2002 to 2015. Of the 31,000 women covered in the survey, 4,184 reported tubal ligation.

In those surveys, between 3 and 5% of the women reported pregnancy after surgery. That is higher than the less than 1% rate typically quoted by health providers and the 18-37 pregnancies per 1,000 women after 10 years estimated by the American College of Obstetricians and Gynecologists.

The data has significant weaknesses: surveys rely on self-reporting from patients, which is generally less reliable than information based on medical records. The survey data did not ask about what type of tubal ligation surgery women believed they had undergone.

Researchers said that if that failure rate is accurate, it would mean tubal ligation is less effective than long-term contraceptive methods such as arm implants and intrauterine devices (IUDs).

The study did hint that the timing of the surgery may matter: women were less likely to report a pregnancy after tubal ligation if the surgery was performed immediately postpartum. Perhaps, Schwarz said, that is because women’s reproductive anatomy is enlarged and more visible immediately after giving birth.

Notably, pregnancy was less likely when women received the surgery when they were older.

The work builds on Bimla Schwarz’s previous research on the effectiveness of tubal ligation. In 2022, she published research based on more than 83,000 Medi-Cal insurance claims, the public health insurance program Medicaid in California. That study found that tubal ligation had roughly the same effectiveness rate as an IUD.

If tubal ligation is as effective as an IUD, authors said it also implies younger women especially may consider an IUD before permanent surgical sterilization, since they are easily reversible. Although most women do not regret the surgery, younger women are significantly more likely to express regret than older women.

“If the number one goal is not to get pregnant in the future, and you’re really interested in the effectiveness”, then a contraception arm implant is probably the best option, said Schwarz. She said vasectomy, or permanent male sterilization, is close behind, followed by hormonal IUDs, tubal ligation and copper IUDs.

Schwarz added: “It’s important for anybody really interested in effectiveness to have really thought about all of those options and talked about all of them with their clinician.”