Luteinizing hormone drops to very low levels during pregnancy, suppressed by the flood of hormones the placenta produces. In a non-pregnant cycle, LH plays the starring role in triggering ovulation, but once conception occurs and the embryo implants, the closely related hormone human chorionic gonadotropin (hCG) effectively replaces LH’s duties. The result is that LH becomes nearly undetectable in maternal blood for most of gestation, and the story of why that happens reveals a surprisingly elegant hormonal handoff.
What LH Does Before Pregnancy
To understand why LH disappears during pregnancy, it helps to know what it normally does. LH is released from the pituitary gland in pulses throughout the menstrual cycle. As a follicle matures in the ovary, rising estradiol levels eventually trigger a massive spike in LH secretion known as the LH surge.1PubMed. The luteinizing hormone surge–the final stage in ovulation induction: modern aspects of ovulation triggering That surge causes the mature follicle to rupture and release an egg. After ovulation, the empty follicle transforms into a temporary structure called the corpus luteum, which pumps out progesterone to prepare the uterine lining for a possible pregnancy.
The LH surge itself is a complex event involving rising estradiol acting on both the hypothalamus and the pituitary. A mathematical model of this process suggests that estradiol modulates how connected certain pituitary cell networks are: when estradiol is low, the network barely fires, but when estradiol climbs high enough, the network amplifies the signal into a full surge of LH release.2PubMed. Pituitary network connectivity as a mechanism for the luteinising hormone surge This is the system that pregnancy shuts down almost entirely.
How hCG Takes Over
If fertilization occurs and the embryo implants in the uterine wall, the developing placenta begins secreting hCG within days. hCG and LH are closely related molecules that bind to the same receptor on cells. In displacement experiments measuring how tightly each hormone grabs onto the shared LH/hCG receptor, both showed quite similar binding affinity, though hCG appeared to bind somewhat more tightly.3Endocrine Reviews. Two Hormones for One Receptor: Evolution, Biochemistry, Actions, and Pathophysiology of LH and hCG Despite their structural similarities, their physiological jobs differ: LH regulates ovarian steroid production and ovulation, while hCG is predominantly active during pregnancy and fetal development.4PubMed Central. Luteinizing hormone and human chorionic gonadotropin: distinguishing unique physiologic roles
The first critical job of hCG is rescuing the corpus luteum. Without a pregnancy signal, the corpus luteum is programmed to break down after about two weeks, which is what triggers a menstrual period. hCG prevents that breakdown, keeping the corpus luteum alive and producing progesterone, which is essential to maintaining the uterine lining and supporting the embryo.5PubMed. Rescue of the corpus luteum in human pregnancy Rising hCG also regulates the blood supply surrounding the corpus luteum, ensuring adequate progesterone output during those early weeks.6Human Reproduction. Effect of rising hCG levels on the human corpus luteum during early pregnancy
Why LH Gets Suppressed
As hCG levels skyrocket during the first trimester, the pituitary gland’s output of both LH and FSH becomes markedly suppressed.7Elsevier / The Clinics. The Maternal Adenohypophysis / Obstetrics and Gynecology Clinics The mechanism behind this involves the same negative feedback loop that regulates LH throughout the menstrual cycle. Progesterone and estrogen, now produced in large quantities first by the corpus luteum and later by the placenta, tell the hypothalamus and pituitary to dial down gonadotropin-releasing hormone (GnRH) pulses and LH secretion. hCG itself, by activating the same receptor as LH, further contributes to the suppressive signal.
The suppression is deep enough that researchers studying patients with molar pregnancies (an abnormal form of pregnancy producing extremely high hCG) found that even injecting the pituitary with GnRH failed to produce a normal FSH response. The pituitary’s gonadotropin-producing cells were, in effect, shut down by an unknown mechanism during both molar and normal pregnancies.8PubMed. Follicle-stimulating hormone response to luteinizing-hormone-releasing hormone in patients with hydatidiform mole So it is not simply that the body stops asking the pituitary to release LH; the pituitary itself becomes resistant to stimulation.
The Luteoplacental Shift
The corpus luteum does not stay in the picture forever. By roughly six to eight weeks of pregnancy, the placenta has grown enough to produce its own progesterone, and the corpus luteum gradually fades away.9PubMed Central. The inadequate corpus luteum This transition from ovarian to placental progesterone production is called the luteoplacental shift. In studies tracking specific steroid markers, researchers found that in normal pregnancies, the ratio of progesterone to a predominantly ovarian steroid product rose between weeks four and nine, reflecting the placenta’s increasing contribution.10PubMed. Delayed luteo-placental shift of progesterone production in IVF pregnancy
Interestingly, this transition can be delayed in IVF pregnancies. In those cases, the ratio did not change over the same period, suggesting the ovary was shouldering the progesterone burden longer than usual.10PubMed. Delayed luteo-placental shift of progesterone production in IVF pregnancy One early model of the luteoplacental shift in women with absent ovarian function (who became pregnant through donor eggs) showed that placental estradiol and progesterone levels rose significantly around 25 and 34 days after embryo transfer, respectively, confirming the approximate timing of when the placenta fully assumes its hormone-producing role.11PubMed. A human in vivo model for the luteoplacental shift
Once the placenta takes over, LH becomes essentially irrelevant to maintaining the pregnancy. The placenta produces its own hCG, progesterone, and estrogen independently. LH stays suppressed for the remainder of gestation.
When Pre-Pregnancy LH Levels Are Already Abnormal
Women with polycystic ovary syndrome (PCOS) often have elevated baseline LH levels even before pregnancy is attempted, which can complicate the picture. Elevated baseline LH has been shown to impede the effectiveness of ovulation induction treatments.12PubMed Central. Successful Pregnancy in a PCOS Patient with Elevated Basal Luteinizing Hormone and Repeated Ovulation Induction Failures: A Case Report More concerning, one study of women with PCOS undergoing ovarian stimulation and intrauterine insemination found a stair-shaped relationship between baseline LH and miscarriage risk: early miscarriage grew significantly once baseline LH exceeded about 11.7 mIU/mL.13Heliyon. Hypersecretion of basal luteinizing hormone and an increased risk of pregnancy loss among women with polycystic ovary syndrome undergoing controlled ovarian stimulation and intrauterine insemination
However, another study examining PCOS patients undergoing IVF found that cumulative clinical pregnancy and live birth rates were not significantly different across groups with varying baseline LH levels.14PubMed Central. Effect of basal luteinizing hormone (bLH) level on in vitro fertilization/intra-cytoplasmic injections (IVF/ICSI) outcomes in polycystic ovarian syndrome (PCOS) patients The discrepancy likely reflects differences in treatment protocols. In IVF, hormone levels are more tightly controlled, the pituitary is often deliberately suppressed with medication, and embryo quality is assessed before transfer. The evidence is mixed enough that researchers have not reached a consensus on a single LH threshold that predicts trouble across all fertility treatment settings.
LH in Assisted Reproduction and Luteal Phase Support
In fertility treatment cycles, the pituitary is frequently suppressed by medications (GnRH agonists or antagonists) to prevent a premature LH surge that could ruin the timing of egg retrieval. The downside is that this artificial suppression can leave the corpus luteum starved for stimulation, leading to inadequate progesterone output in the critical early weeks. That is why luteal phase support is a standard part of most IVF protocols, using approaches that include progesterone supplements, hCG injections, estradiol, GnRH agonists, and recombinant LH.15PubMed Central. Luteal-phase support in assisted reproductive technology: An ongoing challenge
The timing of when recombinant LH is added during stimulation also seems to matter. In one study of patients on long-acting GnRH agonist protocols, adding recombinant LH later in the stimulation cycle, when the dominant follicle was larger, was associated with higher clinical pregnancy rates and lower miscarriage rates.16PubMed Central. Effect of recombinant LH supplementation timing on clinical pregnancy outcome in long-acting GnRHa downregulated cycles This suggests that the window during which LH activity is helpful to the maturing follicle is relatively narrow, and that bombarding it with LH too early may not help or could even be counterproductive.
Postpartum Recovery of LH
After delivery, hCG levels plummet rapidly (it is cleared from the blood within a couple of weeks), and the pituitary’s suppression lifts. How quickly LH bounces back depends heavily on breastfeeding. In mothers who did not breastfeed, LH levels rose relatively quickly and had returned to within normal limits by about three weeks postpartum.17PubMed. Fertility after childbirth: changes in serum gonadotrophin levels in bottle and breast feeding women In breastfeeding mothers, the return was slower and more variable.
The mechanism behind this breastfeeding-related delay is not as straightforward as many assume. A common explanation is that prolactin, the hormone driving milk production, directly suppresses LH. But a study tracking 24-hour patterns of pulsatile LH release during lactational amenorrhea found no difference in prolactin levels or suckling patterns between women who had detectable LH pulses and those who did not.18Human Reproduction. The 24 h pattern of pulsatile luteinizing hormone, follicle stimulating hormone and prolactin release during the first 8 weeks of lactational amenorrhoea in breastfeeding women That finding suggests prolactin alone is not the whole story, and that other signals related to breastfeeding, possibly acting at the hypothalamic level, are also involved in keeping LH suppressed. Regardless of the exact mechanism, the practical result is familiar: breastfeeding delays the return of ovulation and menstruation, and suppressed LH is the proximate cause.
Beyond the Ovary: What LH Does to Other Tissues
Researchers have increasingly recognized that LH does not just act on the ovary. One study examining endometrial stem cells found that LH suppressed various tissue regeneration functions in those cells, including self-renewal, migration, and differentiation potential, by inhibiting certain pro-survival signaling pathways.19Cell Death & Disease. Novel roles of luteinizing hormone (LH) in tissue regeneration-associated functions in endometrial stem cells One consequence of this was decreased endometrial receptivity. The finding is provocative because it suggests that LH suppression during pregnancy might actually benefit the endometrium by removing a brake on its regenerative capacity, though this line of research is still early-stage and mostly based on laboratory experiments rather than clinical studies in pregnant women.
How Primates Evolved a Replacement for LH
The fact that hCG exists at all is a quirk of primate evolution. The gene coding for hCG’s unique subunit arose by duplication of the LH gene, and this happened only in primates, sometime between 55 and 35 million years ago.20Encyclopedia of Life Sciences. Evolution of the Chorionic Gonadotropin β Genes in Primates A detailed analysis of 14 primate and related mammalian species found that bats, flying lemurs, lower primates, and tarsiers all lack a separate hCG gene, possessing only one copy of the LH gene. The hCG gene first appeared in the common ancestor of higher primates, after the lineage split from tarsiers, and subsequent duplication events gave some primates multiple copies.21PubMed. Chorionic gonadotropin has a recent origin within primates and an evolutionary history of selection
Only a handful of sequence changes in the duplicate gene were needed to produce a molecule with different tissue expression and function from LH. In evolutionary terms, that is a remarkably elegant innovation: a small tweak that gave primate embryos the ability to directly signal to the mother’s ovary and secure the progesterone needed for implantation and early pregnancy support.
How Other Mammals Manage Without hCG
If hCG is unique to primates, how do other mammals keep their corpus luteum alive during pregnancy? The strategies vary enormously. In elephants, which gestate for 22 months, the system is entirely LH-dependent. Elephants produce two LH peaks during their follicular phase rather than one, which results in the formation of multiple corpus luteum structures. After about five to six weeks these structures begin to regress, but once an embryo becomes visible on ultrasound around day 49, all pregnancy corpus luteum structures resume growing and persist until after birth.22PubMed Central. Gestating for 22 months: luteal development and pregnancy maintenance in elephants The elephant corpus luteum, in other words, stays dependent on pituitary LH throughout the entire pregnancy, never being “rescued” by a placental hormone the way it is in primates.
Ruminants like cows and sheep use yet another approach. Their large luteal cells grow bigger than their primate counterparts and develop a capacity for high, constitutive progesterone production that does not require ongoing LH stimulation at all.23PubMed Central. Comparison of endocrine and cellular mechanisms regulating the corpus luteum of primates and ruminants Essentially, the ruminant corpus luteum becomes an autonomous progesterone factory. These different evolutionary solutions to the same problem, keeping progesterone flowing long enough to sustain a pregnancy, highlight just how unusual the primate strategy of inventing an entirely new hormone really is. And they underscore why, in humans, LH can afford to vanish from the bloodstream for nine months: hCG has it covered.