Follicle-Stimulating Hormone (FSH) is a protein hormone produced by the pituitary gland, a small organ located at the base of the brain. Its primary function is to regulate the reproductive system in both males and females, though it is most commonly discussed in the context of a woman’s fertility. Once a woman successfully conceives, the body’s hormonal environment shifts dramatically, leading to the suppression of FSH. For this reason, FSH is typically not measured or monitored during a healthy, established pregnancy because its work, in terms of stimulating an egg, is complete.
FSH’s Role in Fertility and Ovarian Reserve
FSH’s name directly reflects its job in the non-pregnant state: stimulating the ovarian follicles, which are tiny sacs that contain immature eggs. At the start of a menstrual cycle, FSH levels rise to encourage a cohort of follicles to begin growing and maturing. As these follicles develop, they produce the hormone estrogen, which is necessary to prepare the uterine lining for potential implantation.
The level of FSH in the blood, measured in the early follicular phase (around day three of the menstrual cycle), is a long-standing tool used to assess a woman’s ovarian reserve—the quantity and quality of remaining eggs. High basal FSH levels suggest the ovaries are not responding effectively and require a greater “push” from the pituitary gland to stimulate a follicle.
A normal basal FSH level is generally considered to be less than 10 milli-international units per milliliter (mIU/mL). Levels consistently above this range, especially those over 15 mIU/mL, indicate diminished ovarian reserve, signaling that the pituitary is working harder to stimulate the ovaries, which are becoming less responsive.
While FSH testing is a component of fertility assessments, it is rarely interpreted in isolation. Fertility specialists consider it alongside other markers, such as Anti-Müllerian Hormone (AMH) and the Antral Follicle Count (AFC), for a complete picture of reproductive potential. Understanding the role of FSH outside of pregnancy provides context for why its presence or absence during gestation is so significant.
Expected FSH Levels During Healthy Pregnancy
During a healthy pregnancy, the expected FSH level is extremely low, often dropping to undetectable levels. The shift from a reproductive cycle to a gestational state involves a profound hormonal takeover that suppresses the pituitary gland’s need to produce FSH. This suppression is a completely normal and necessary physiological response.
Suppression involves two key pregnancy hormones: estrogen and progesterone. These hormones are produced at very high and sustained concentrations, first by the corpus luteum and later by the placenta. The elevated levels of estrogen and progesterone provide a strong negative feedback signal to the pituitary gland and hypothalamus.
This signal tells the brain the body is pregnant, eliminating the need to stimulate new follicles or prepare for another ovulation cycle. The pituitary gland halts the secretion of FSH and Luteinizing Hormone (LH). FSH levels are often reported as less than 0.3 mIU/mL throughout pregnancy, confirming this profound suppression.
Human Chorionic Gonadotropin (hCG), the hormone detected by pregnancy tests, takes over a function previously regulated by LH. hCG maintains the corpus luteum, the temporary ovarian structure that produces progesterone until the placenta is fully developed. This ensures a steady supply of pregnancy-sustaining hormones, making the stimulation of new follicles by FSH unnecessary.
Clinical Scenarios Where FSH Testing is Relevant
FSH testing is not a part of routine prenatal care because its levels are expected to be profoundly low. However, FSH testing may be ordered in specific clinical situations related to a pregnancy concern. The most common scenario is its use in the pre-conception phase as part of a comprehensive fertility workup.
A less common, but relevant, situation is when a pregnancy is not progressing normally, such as in suspected miscarriage or ectopic pregnancy. If pregnancy hormones like hCG and progesterone suddenly drop due to pregnancy loss or abnormal implantation, the negative feedback signal to the pituitary gland is removed. This sudden drop might allow FSH levels to prematurely begin to rise.
While a rising FSH level is a possible indicator of hormonal failure, it is not the primary diagnostic tool. Clinicians rely on monitoring the trend of hCG and progesterone levels, often alongside ultrasound, to assess pregnancy viability. Testing FSH in these circumstances serves as a secondary confirmation that the hormonal environment is reverting to a non-pregnant state.
FSH may also be checked in rare instances of unexplained bleeding or suspected pituitary dysfunction during pregnancy. In almost all cases, the FSH level during a confirmed, healthy pregnancy should be viewed as inconsequential because its role has been successfully superseded by the powerful hormonal milieu of gestation.