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Overview
The progesterone in birth control pills creates a thick cervical mucus, making it difficult for sperm to reach the uterus. It also impedes an egg from attaching itself to the uterine lining (endometrium) because of changes in the cellular structure of the lining.
Review Date 1/10/2022
Updated by: John D. Jacobson, MD, Department of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.