Progesterone is a naturally occurring steroid hormone central to the female reproductive system. It is produced by the body and widely prescribed in various synthetic or bioidentical forms for medical treatment. This prescription use of progesterone and its synthetic counterparts, known as progestins, is employed for two opposing purposes: preventing pregnancy and supporting it. Whether a person can conceive while taking this hormone depends entirely on the specific reason it was prescribed.
Progesterone’s Essential Role in Reproduction
The body naturally produces progesterone primarily in the ovaries after ovulation, specifically from the corpus luteum. This hormone converts the uterine lining, or endometrium, from a proliferative state to a secretory state. This transformation makes the endometrium thick, receptive, and nutrient-rich, preparing it to sustain a fertilized egg for implantation.
If fertilization and implantation occur, the corpus luteum continues to secrete high levels of progesterone, maintaining the uterine lining and preventing menstruation. Progesterone also works to keep the uterine muscles relaxed, helping to prevent premature contractions in early pregnancy. If pregnancy does not occur, the corpus luteum breaks down, causing progesterone levels to fall rapidly, which triggers the shedding of the uterine lining. High levels of the hormone during the latter half of the cycle also suppress the release of further eggs.
Progesterone Used for Contraception
When progesterone-like substances are used for birth control, they are almost always synthetic hormones called progestins, modified to be more potent and effective as contraceptives. Progestin-only methods, such as the mini-pill, implants, and injections, primarily prevent conception through a dual mechanism. The first action is thickening the cervical mucus, creating a physical barrier that sperm cannot penetrate to reach the egg.
The second mechanism is suppressing the release of an egg, or ovulation, which is how higher-dose progestins or newer formulations work. With typical use, the progestin-only pill (POP) is about 93% effective, meaning about seven pregnancies occur per 100 users in a year. Long-acting progestin methods, like injections or implants, are highly effective, with a failure rate of less than one pregnancy per 100 users annually.
A high-dose progestin, specifically levonorgestrel, is the active ingredient in many forms of emergency contraception. This medication, sometimes known as Plan B, works primarily by preventing or delaying ovulation. It is most effective when taken soon after unprotected sex and does not work once implantation has begun. Although breakthrough pregnancy is rare with these methods, the chance remains, particularly if user error occurs or if the method is less effective for an individual.
Progesterone Used to Support Conception
The therapeutic use of natural progesterone is entirely different, prescribed specifically to create an optimal environment for conception and continuation of a pregnancy. This is common in assisted reproductive technology (ART) cycles, such as in vitro fertilization (IVF), where supplementation is referred to as Luteal Phase Support. Ovarian stimulation drugs used in IVF can impair the body’s ability to produce progesterone, making supplementation necessary to prepare the uterus for embryo implantation.
Progesterone is also prescribed for individuals who have experienced recurrent pregnancy loss or bleeding in early pregnancy. The hormone stabilizes the uterine lining, supporting early gestation. In these cases, progesterone is not causing fertilization, but rather supporting endometrial receptivity and maintenance of the pregnancy once conception has occurred. Progesterone is often administered vaginally or via injection, as oral absorption is generally insufficient to achieve the necessary levels for uterine support.
Practical Guidance When Pregnancy is Suspected
Individuals taking progesterone for fertility support or to prevent miscarriage must be aware that the hormone can mimic or mask the signs of early pregnancy. Since high progesterone levels prevent a period from starting, the medication may delay or prevent menstrual bleeding even if pregnancy has not occurred. Therefore, a missed or delayed period cannot be reliably used as the sole sign of pregnancy.
The most accurate way to confirm a suspected pregnancy is through a blood or urine test that detects human chorionic gonadotropin (hCG). Progesterone does not interfere with these tests and will not cause a false positive result. If a positive pregnancy test occurs, it is imperative to contact the prescribing physician immediately. Abruptly stopping progesterone, especially when used for fertility treatments or recurrent loss, can be detrimental to a developing pregnancy, so a healthcare provider must determine the correct course of action.