The Birth Control Pill Blocks Pregnancy Three Ways at Once
Even if an egg slips through, thicker mucus and a thinner uterine lining provide backup.
HEALTH TALK AMERICA · Sugar, Antibiotics, and the Trillions of Bugs Running Your Immune SystemThe birth control pill blocks pregnancy in three ways at once. It stops ovulation, it thickens cervical mucus to block sperm, and it thins the uterine lining.
Dr. Adam Brockman, a physician, explained it this way. There are two main formulas, combination pills that contain synthetic estrogen and progestin, and progestin-only pills. He said they work in a few different ways.
The first way involves the ovaries. In a typical cycle, an ovary releases a mature egg, an event called ovulation, and that egg could meet sperm. The pill stops ovulation, so there is no egg available to meet sperm.
The second way involves the cervix, the narrow opening to the uterus. The mucus there normally thins around ovulation to help sperm swim through. The pill thickens that mucus into a stickier barrier, which makes it harder for sperm to travel.
The third way involves the uterine lining, the layer inside the uterus. The pill keeps that lining thin, so a fertilized egg would have a harder place to settle.
Taken correctly, Brockman said, the pill is about 99 percent effective. He added that in real life, missed pills and late doses lower that protection.
Questions people ask
What is the difference between combination pills and progestin-only pills?
Combination pills contain synthetic versions of both estrogen and progestin, while progestin-only pills contain only progestin. Progestin-only pills are often prescribed for people who cannot take estrogen because of migraines with aura, high blood pressure, clotting risk, or breastfeeding.
Does thinning the uterine lining cause lasting infertility?
No. The thickening and thinning reverse after the hormones leave the system, and ovulation usually resumes. Brockman advised having another birth control plan ready before stopping, because pregnancy can happen at any time, although cycles usually take a few months to settle back in.
What should I do if I miss a pill?
Protection drops because hormone levels fall, which can allow ovulation to restart and cervical mucus to thin. What to do depends on how many pills were missed and when in the pack it happened, so the package insert and a pharmacist or clinician should guide backup contraception and catch-up dosing.