Getting pregnant depends on a carefully timed chain of at least six hormones working together. Some build the egg, some trigger its release, some prepare the uterus, and others keep a new pregnancy alive in its earliest days. When any one of these hormones is too high, too low, or mistimed, conception becomes harder or fails entirely.
GnRH: The Master Signal
Everything starts in a small region of the brain that releases gonadotropin-releasing hormone (GnRH) in rhythmic pulses. These pulses act like a conductor setting the tempo for the entire reproductive cycle. The timing matters: pulses roughly every 60 minutes during the first half of your cycle keep the downstream hormones at the right levels. When researchers slowed those pulses to every 120 minutes, women had significantly lower levels of luteinizing hormone (LH), a blunted midcycle hormone surge, and reduced rates of ovulation. In other words, if this brain signal fires too slowly, the whole chain stalls before it really begins.
FSH and LH: Growing and Releasing the Egg
GnRH pulses tell the pituitary gland to release two hormones that act directly on the ovaries: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). They have distinct but overlapping jobs.
FSH does what its name suggests. It stimulates the small fluid-filled sacs (follicles) in the ovary, promoting cell division and helping one follicle mature into the dominant one that will release an egg. FSH also drives estrogen production inside that follicle by activating an enzyme system that converts other hormones into estrogen. Without adequate FSH, follicles stall at an early stage and no mature egg develops.
LH works alongside FSH but takes a leading role in later follicle growth. It promotes the production of growth factors that increase follicle size and drive the structural changes needed before ovulation. LH also stimulates cells surrounding the follicle to produce the raw hormonal building blocks that FSH then converts into estrogen. The most dramatic moment comes mid-cycle: a sharp spike in LH, called the LH surge, triggers the follicle to rupture and release the egg. If the midcycle LH surge is blunted or absent, ovulation typically does not occur.
Estrogen: Preparing the Path for Sperm and Embryo
As the follicle grows, it produces increasing amounts of estrogen (specifically estradiol). This hormone has two critical jobs before ovulation even happens.
First, rising estrogen transforms cervical mucus into a clear, slippery, egg-white consistency that allows sperm to swim through the cervix and reach the egg. Earlier in the cycle, when estrogen is low, cervical mucus is thick and acts as a barrier. This shift in mucus quality is one of the most reliable signs that ovulation is approaching, and it is entirely estrogen-driven.
Second, estradiol thickens the uterine lining (endometrium) during the first half of the cycle. Research on fertility treatments shows that pregnancy rates increase gradually as lining thickness grows, with the best outcomes at around 10 millimeters or above. A lining under 7 millimeters is considered thin and associated with lower chances of pregnancy. Higher estradiol levels at the time of ovulation have also been linked to better pregnancy rates, reinforcing that this hormone needs to reach sufficient levels for conception to succeed.
Progesterone: Holding the Pregnancy in Place
After the egg is released, the empty follicle transforms into a temporary gland called the corpus luteum, which begins producing progesterone. This hormone is arguably the single most important one for maintaining early pregnancy. It converts the estrogen-thickened uterine lining into a spongy, nutrient-rich environment that a fertilized egg can implant into and grow.
Normal progesterone levels during the luteal phase (the two weeks after ovulation) range from 2 to 25 ng/mL, rising to 10 to 44 ng/mL during the first trimester if pregnancy occurs. When midluteal progesterone falls below 5 ng/mL, that is considered biochemical evidence of luteal phase deficiency, a condition that affects roughly 8 to 9 percent of cycles even in women who menstruate regularly. The American Society for Reproductive Medicine defines this condition as a luteal phase lasting 10 days or fewer, meaning progesterone drops off too soon for an embryo to successfully implant. Since progesterone is required for a normal intrauterine pregnancy, there is almost certainly a threshold below which pregnancy cannot be established or maintained.
hCG: The Rescue Signal After Conception
If a fertilized egg implants in the uterine lining, it immediately begins producing human chorionic gonadotropin (hCG). This hormone has one urgent job: keeping the corpus luteum alive so it continues making progesterone. Without hCG, the corpus luteum would break down about two weeks after ovulation, progesterone would plummet, the uterine lining would shed, and the pregnancy would end before it truly started.
The corpus luteum sustains progesterone production for about 12 weeks of pregnancy, at which point the placenta takes over. hCG is also the hormone detected by pregnancy tests, which is why very early pregnancies sometimes produce faint lines: hCG levels are still climbing.
Thyroid Hormone and Prolactin: The Background Players
Two hormones that are not part of the reproductive system itself can quietly interfere with fertility when they are out of range.
Thyroid-stimulating hormone (TSH) is often checked during a fertility workup. While the standard normal range extends up to about 5 mIU/L, research from a large academic health system found that women with TSH levels above 2.5 mIU/L were more likely to have unexplained infertility compared to women with lower levels. Some experts now argue that 2.5 mIU/L should be considered the upper limit of normal for women trying to conceive, since 95 percent of people without thyroid disease fall below that number.
Prolactin, the hormone that stimulates milk production, can suppress ovulation when it is elevated. Levels at or below 20 ng/mL are considered normal. High prolactin outside of breastfeeding can disrupt GnRH pulses, which in turn lowers FSH and LH and prevents regular ovulation.
AMH: Gauging Your Egg Supply
Anti-Müllerian hormone (AMH) does not directly cause pregnancy, but it reflects how many eggs remain in your ovaries, a measurement called ovarian reserve. AMH peaks around age 25 and declines steadily from there. General benchmarks on the lower end of the spectrum by age:
- Age 25: 3.0 ng/mL
- Age 30: 2.5 ng/mL
- Age 35: 1.5 ng/mL
- Age 40: 1.0 ng/mL
- Age 45: 0.5 ng/mL
Average AMH falls between 1.0 and 3.0 ng/mL. Levels under 1.0 ng/mL are considered low, and below 0.4 ng/mL is severely low. AMH is often tested alongside day 3 FSH during initial fertility evaluations. An elevated day 3 FSH level, or a high ratio of FSH to LH on day 3, can signal diminished ovarian reserve even when FSH alone looks normal. Research suggests that when day 3 LH drops below 3 mIU/mL, ovarian activity may be reduced enough to lower the number of follicles that reach maturity.
How These Hormones Work as a Sequence
What makes fertility hormones tricky is that they are not independent. They operate as a tightly linked sequence where each one triggers the next. GnRH pulses release FSH and LH. FSH and LH grow the follicle and drive estrogen production. Rising estrogen thickens the lining and changes cervical mucus. The estrogen peak triggers the LH surge. The LH surge causes ovulation. The empty follicle becomes the corpus luteum and produces progesterone. Progesterone prepares the lining for implantation. If an embryo implants, hCG rescues the corpus luteum so progesterone keeps flowing.
A disruption at any point, whether it is sluggish GnRH pulses, insufficient progesterone after ovulation, elevated TSH quietly suppressing the cycle, or low AMH signaling a shrinking egg supply, can prevent pregnancy even when everything else is working. This is why fertility evaluations typically test several of these hormones at specific points in the cycle rather than relying on any single number.