Background: In postpartum women who are breastfeeding, progestin-only oral contraceptive pills (OCPs) traditionally have been recommended because of concerns about the effect of combination OCPs on milk supply. However, compared with progestin-only OCPs, combination OCPs are more effective, have fewer adverse effects, and are associated with a higher breastfeeding continuation rate. Espey and colleagues compared the effects of progestin-only and combined OCPs on breastfeeding rates and infant growth at eight weeks postpartum.

The Study: This single-center, double-blind, randomized controlled trial enrolled women at the University of New Mexico’s prenatal clinics from January 2005 until June 2008. Women 15 to 45 years of age were eligible if they intended to breastfeed and use OCPs. Exclusion criteria included contraindications to estrogen-containing pills (e.g., a history of venous thromboembolism, complex migraine headaches, uncontrolled hypertension), preterm birth, small- or large-for-gestational-age newborn, and a newborn with a major congenital anomaly. Baseline data were collected, including maternal smoking and breastfeeding history, as well as newborn length, weight, and head circumference.

Women who were still breastfeeding at two weeks postpartum were randomized to start eight weeks of progestin-only or combined OCPs. The progestin-only pill contained the standard dose of 0.35 mg of norethindrone, whereas the combined pill contained 1 mg of norethindrone and 0.035 mg of ethinyl estradiol. Both cohorts received active pills for 21 days, and all participants received the identical placebo for seven days. The pills were packaged in identical capsules and blister packs. Research nurses contacted participants weekly by telephone to assess rates of and satisfaction with breastfeeding, formula supplementation rates, and satisfaction with their contraceptive. All mothers and newborns had a follow-up visit at eight weeks at which participants completed a questionnaire and infant growth parameters were recorded. Participants were given another four-month supply of OCPs, and completed a follow-up telephone survey at four and six months postpartum.

The primary outcome of the study was breastfeeding continuation in women at eight weeks postpartum. Secondary end points included breastfeeding rates at four and six months postpartum, and secondary outcomes included infant weight and length at eight weeks, as well as continuation and satisfaction with the contraceptive method. The eight-week time frame was chosen because it was assumed that any negative effects of combined OCPs on breast milk or breastfeeding would be evident by this time.

Results: At the two-week postpartum visit, 127 women were randomized to receive progestin-only (n = 63) or combined OCPs (n = 64). At this point, the groups had similar rates of exclusive breastfeeding (average of 64 percent) and perceived rates of inadequate milk supply (average of 22 percent). At eight weeks, similar percentages of women were still breastfeeding (approximately 64 percent in both groups). There was no difference in infant growth parameters. For those who stopped breastfeeding, perceived lack of milk supply was the most common reason for discontinuing. However, there was no significant difference between groups (55 percent of those in the combined OCP group versus 44 percent in the progestin-only OCP group stopped breastfeeding because of a perceived lack of milk supply; P = .80). Similarly, there was no difference between groups for those who stopped taking their OCP because of milk supply concerns (23 percent of progestin-only versus 21 percent of combined OCP users). At six months, there was no difference in breastfeeding rates between the two groups.

Conclusion: In this small study, combined OCPs and progestin-only OCPs resulted in similar rates of breast-feeding at eight weeks and at six months, with equal infant growth at eight weeks. Larger studies are needed to confirm the compatibility of combined OCPs with breastfeeding.