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Related Concept Videos

Ovarian Cycle01:27

Ovarian Cycle

The menstrual cycle includes a critical component known as the ovarian cycle, which undergoes two main phases each month—the follicular phase and the luteal phase. The follicular phase is variable and averaging around 14 days. Ovulation, triggered by a surge in luteinizing hormone (LH), marks the transition between the two phases. The second phase, the luteal phase, is relatively consistent, lasting approximately 14 days, and is marked by the activity of the corpus luteum. While a cycle length...
Gonadal and Placental Hormones01:24

Gonadal and Placental Hormones

The gonads, namely the testes in males and the ovaries in females, are pivotal in producing gonadal hormones that orchestrate the intricate processes of sexual development and reproduction.
In males, testosterone is the primary gonadal androgen. It plays a central role in the maturation of male reproductive organs — the penis and testes. Additionally, testosterone is instrumental in the development of secondary sexual characteristics — a deep voice as well as facial and pubic hair growth — and...
Secretory Phase01:19

Secretory Phase

The secretory phase of the menstrual cycle, spanning from day 14 to 28 in a typical 28-day cycle, is a period of significant physiological changes in the female reproductive system. This phase commences immediately after ovulation and is characterized by the preparation of the endometrium for potential embryo implantation.
Following ovulation, the corpus luteum, a temporary endocrine structure, produces progesterone and estrogens. These hormones stimulate the growth and coiling of endometrial...
Hormonal Control of the Ovarian Cycle01:30

Hormonal Control of the Ovarian Cycle

The ovarian cycle is meticulously regulated by the hypothalamic-pituitary-gonadal axis. This cycle orchestrates the release of a mature oocyte, essential for reproduction.
Before puberty, the hypothalamus releases GnRH in a low frequency, low amplitude pulsatile manner. This along with the immature hypothalamic-pituitary-gonadal axis activity, results in low estrogen levels and the absence of a fully functional ovarian cycle.  At puberty, GnRH secretion increases in both frequency and...
Proliferative Phase01:20

Proliferative Phase

The proliferative phase typically occurs after menstruation and lasts between 6 to 13 days in a standard 28-day cycle. This phase involves the reconstruction of the endometrium, guided by estrogen produced by the developing ovarian follicle.
Notably, the stratum basale, the basal layer of the endometrium, including the basal parts of the uterine glands, remains unaffected by menstruation. Stem cells in this layer undergo mitosis, regenerating the stratum functionalis and thickening the...
Hormonal Regulation of the Menstrual Cycle01:22

Hormonal Regulation of the Menstrual Cycle

The ovarian cycle regulates endometrial changes throughout a single menstrual cycle via the coordinated action of gonadotrophin-releasing hormone (GnRH) and gonadotrophins.
At puberty, GnRH begins a pulsatile release pattern, which triggers the anterior pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The frequency and amplitude of GnRH pulses vary across the menstrual cycle, with faster pulses favoring LH release and slower pulses favoring FSH release.

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Related Experiment Video

Updated: Jul 15, 2026

Methods for Studying Uterine Contributions to Pregnancy Establishment in an Ovariectomized Mouse Model
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Methods for Studying Uterine Contributions to Pregnancy Establishment in an Ovariectomized Mouse Model

Published on: April 7, 2023

Progesterone withdrawal: key to parturition.

Tamas Zakar1, Frank Hertelendy

  • 1Division of Obstetrics and Gynaecology, John Hunter Hospital, University of Newcastle, Newcastle, NSW, Australia.

American Journal of Obstetrics and Gynecology
|April 4, 2007
PubMed
Summary

Progesterone withdrawal is crucial for labor initiation in many mammals, but its mechanism in humans remains unclear. This review explores functional progesterone withdrawal, bridging gaps between human and animal pregnancy research.

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Area of Science:

  • Reproductive biology
  • Endocrinology
  • Mammalian parturition

Background:

  • Progesterone is essential for maintaining pregnancy.
  • Progesterone withdrawal triggers labor in most subprimate mammals via luteolysis or altered steroidogenesis.
  • Unlike subprimates, humans do not exhibit a significant drop in progesterone before labor.

Purpose of the Study:

  • To review the historical concept of progesterone withdrawal in pregnancy.
  • To discuss current theories on functional progesterone withdrawal in humans.
  • To reconcile differences in parturition regulation between humans and subprimate mammals.

Main Methods:

  • Literature review of historical and recent research.
  • Comparative analysis of progesterone regulation in human and subprimate pregnancies.
  • Discussion of steroidogenesis and luteolysis mechanisms.

Main Results:

  • The "progesterone block" concept is primarily based on animal models.
  • Human pregnancy does not show a clear progesterone withdrawal before labor.
  • Functional progesterone withdrawal offers a potential explanation for human parturition.

Conclusions:

  • The regulation of parturition differs significantly between humans and subprimate mammals.
  • Understanding functional progesterone withdrawal is key to explaining human labor.
  • Further research is needed to fully elucidate progesterone's role in human parturition.