Just had a baby, and not ready for another one quite yet?

To be safe, you should consider using contraception as soon as 3 weeks after birth, according to a new review published in Obstetrics & Gynecology.

Women who are breastfeeding are very unlikely to conceive, and most women who aren’t breastfeeding won’t start ovulating again until 6 weeks after giving birth. Still, it’s possible in less time, say the authors.

“For women with a new baby, contraception may not be at the top of their list of concerns,” Dr. Emily Jackson, one of the study’s authors, from the World Health Organization (WHO), told Reuters Health in an email.

“It is really important that people who provide care to postpartum women bring up the subject of contraceptives, alert women to the fact that they may become fertile soon after having a baby, and make sure that women have their chosen method before they become fertile again,” said Jackson, also a family doctor in Los Angeles.

Using some kinds of contraceptive pills right after pregnancy is dangerous because both the estrogen in pills and post-pregnancy hormones increase a woman’s risk of blood clots. That risk drops off over time.

The aim of the current study was to help determine at what point after a woman gives birth the benefits of using contraceptive pills again begin to outweigh the risks.

Jackson and her colleague Dr. Anna Glasier reviewed four studies that have examined when non-breastfeeding women begin to ovulate again after giving birth, and whether women had a good chance of getting pregnant during those first ovulations.

In all of the studies combined, ovulation started, on average, between 45 and 94 days after a woman gave birth. However, in two studies women started ovulating as early as 25 and 27 days after giving birth.

The studies also found that most of those first ovulations probably wouldn’t result in pregnancy.

Based on these results, and on data regarding the likelihood of blood clots, the WHO determined that the benefits of starting contraceptive pills containing both estrogen and progestin probably outweigh any risks starting at 3 weeks after birth.

After 6 weeks, WHO researchers said that there should be no restrictions on new mothers taking contraceptive pills.

Contraceptive pills that contain progestin only are thought to be safe right away after a woman gives birth, and so these could be an option for women, said Dr. Kavita Nanda, a researcher at Family Health International who was not involved in the current study.

The study’s recommendations only apply to women who are not regularly breastfeeding.

In addition, doctors don’t recommend that mothers who are breastfeeding take contraceptive pills with estrogen, because of a controversial potential risk that those could slow infants’ growth.

Jackson also said it’s important that doctors speak to all women, including women who are breastfeeding, about their options for contraception.

“Breastfeeding can be a lot of work, and women may have a break in breastfeeding that they don’t plan for or stop breastfeeding earlier than they thought they would, potentially putting them at higher risk for pregnancy unexpectedly,” Jackson said.

“It would be great if we could make sure that all women were prepared in advance to address their return to fertility postpartum.”

SOURCE: Obstetrics & Gynecology, March 2011.