The normal menstrual cycle in women

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Abstract

The menstrual cycle in women is characterised by high variability in cycle length (26–35 days), 5-day menses, a fertile phase from 5 days before to the day of ovulation, and low fertility which is dependent on cycle length and age. All women show an FSH rise at the luteal–follicular transition, stimulating a cohort of follicular growth and inhibin B secretion in the early follicular phase. The ovulatory dominant follicle (DF) is selected in the mid-follicular phase, and as this DF grows it increasingly secretes oestradiol and inhibin A for a week before ovulation. Gonadotrophin responsiveness, IGF binding protein expression and degradation, and vascularisation have been identified to be crucial for DF selection and progression. Two-thirds of women show two follicle waves and 1/3 show 3 follicle waves per cycle. Three-wave women have longer cycles, and a later oestradiol rise and LH surge. The corpus luteum secretes progesterone, oestradiol and inhibin A in response to LH pulses, and reaches its peak in terms of size, secretions, and vascularization 6–7 days after ovulation. Luteal regression is passive and independent of the uterus, but can be prevented by hCG, the luteotrophic signal from the trophoblast, from 8 days after conception. Reductions in systemic steroid and protein hormone concentrations may be responsible for the FSH rise characteristic of premenopausal women. The functional layer of the endometrium shows steroid hormone-dependent proliferation, differentiation, and shedding in the absence of the trophoblast. Menstruation is initiated by progesterone responsive decidual cells, and executed by PGE and PGF2α, vasoconstriction and matrix metalloprotease secretion by leukocytes. Ovarian function and also hormone fluctuations during the menstrual cycle are similar to oestrous cycles of cows and mares, justifying research into comparative aspects of menstrual and oestrous cycles in monovulatory species.

Introduction

The menstrual cycle of women is tightly controlled by endocrine, autocrine and paracrine factors regulating ovarian follicular development, ovulation, luteinisation, luteolysis, and remodelling of the endometrium. While fundamental reproductive processes are clearly shared between women and our large domestic animals, cycle characteristics, regulatory aspects and research focus can differ widely. For example, cycle irregularities and lack of ovulation are very common in the postpubertal adolescent woman (Golden and Carlson, 2008), but would not have been acceptable in breeding animals, and reduced fertility during reproductive aging is a major research focus in women but not in domestic animals, which are not usually bred late in life.

However, human and animal reproductive physiologists have common interests: (1) the precise detection of the time of ovulation, and (2) understanding determinants of fertility. The first is a challenge to human reproductive medicine, as in menstrual cycles of higher primates ovulation appears ‘hidden’, and menstrual flow does not necessarily indicate normality of cyclicity (Buffet et al., 1998, Harlow, 2000). The second is crucial to achieving economic return from domestic or for conserving wild animal species, but in humans a psychological dimension is added, when couples planning a pregnancy are faced with infertility. Because understanding the menstrual cycle of women will help identify conserved mechanisms underlying ovarian and uterine function, the aim of this review is to discuss what is a normal menstrual cycle, oestrus and fertility; summarise the underlying follicular and luteal function; address the steroid-dependent dynamics of endometrial growth, differentiation and shedding during menstruation; and describe the impact of normal ‘aging’.

Section snippets

The menstrual cycle

Women have a long reproductive lifespan of an average 36 years, from menarche at 8.5 to 13 years to menopause (defined as 1 year of anovulation) at around 51 years (Fig. 1; Aydos et al., 2005, Harlow, 2000, Park et al., 2002). Puberty takes 2.3 years, beginning with breast development followed by pubic hair development and axillary hair growth, and concluding with menarche, the first menstrual period (Park et al., 2002). The ‘textbook’ menstrual cycle (interval from first day of menses to begin

Oestrus

Women have ‘fertile sexuality’ (equated to oestrus) associated with the end of the follicular phase and rising oestradiol before ovulation, which is distinct from ‘extended sexuality’ after ovulation (Gangestad and Thornhill, 2008). During the fertile phase in women features indicative of masculinity and genetic benefit show increased ratings, attraction to extra-pair men increases, while attraction to men who appear sexually faithful decreases. Women's ratings of features indicative of

Fertility

Fertility (clinically diagnosed pregnancy) per menstrual cycle in young women (mean age of 31 years) planning a pregnancy is low (25–30% in the first two cycles) with a high percentage (31%) of subclinical (transient human chorionic gonadotrophin, hCG, rise) and clinical (following a medical diagnosis) spontaneous abortions (Small et al., 2006). The maximum probability of conception to intercourse during the follicular phase is one day before ovulation, however, the fertile-phase spans six

Follicle dynamics

Women ovulate a single DF similar to cows and mares (Fig. 2; Ginther et al., 2001). In reproductive-age women the ovulatory DF emerges at 6 mm diameter on Day 3 of the cycle, 11–12 days before the next ovulation (Ginther et al., 2004), in a cohort of small antral follicles (a follicle wave undergoing ‘cyclic recruitment’, McGee and Hsueh, 2000). Deviation (the time-point from when on DF are morphologically larger than the remaining cohort follicles which will become atretic) occurs 3–4 days

Endocrine changes: the gonadotrophins FSH and LH

As described for cows and mares, a close functional coupling between follicle wave growth, DF selection and FSH exists also in women (Ginther et al., 2001). In regularly cycling women FSH increases at the luteal–follicular transition, beginning 4 days before menses (Fig. 2, Fig. 3; Miro and Aspinall, 2005). Concentrations of FSH achieve maximum levels on the day of emergence of the ovulatory DF followed by a slow decline during the follicular phase (from Days 5 to 13), reaching nadir

Ovarian endocrine function: follicular and luteal secretions

The oestradiol rise in the follicular phase begins after emergence of the ovulatory DF, becomes more rapid following DF selection, and occurs earlier in women with 2 versus 3 follicle waves per cycle (Fig. 2; Baerwald et al., 2003b, Ginther et al., 2005). After ovulation, oestradiol concentrations increase to the mid-luteal phase (Days 7–9 after ovulation) and then decline, and this is due to luteal oestradiol secretion and is unaffected by minor or major anovulatory waves (Fig. 3; Baerwald et

Molecular regulation of dominant follicle selection, ovulation and follicle atresia

Multiple studies have addressed the endocrine, intra-ovarian and intracellular mechanisms regulating follicle growth, differentiation, atresia, ovulation and luteinization in the different monovulatory species including the woman. Key mechanisms involved in dominant follicle selection in the woman, cow and mare appear to be (1) the regulation of the gonadotrophin response, by upregulating LH-receptor and downregulating FSH-receptor expression, (2) ensuring maximum intra-follicular levels of

Molecular aspects of luteal function, regression and rescue by the trophoblast

In contrast to our large animal species luteal progesterone production is dependent on LH throughout the luteal phase, and the result of interactions between steroids, prostaglandins, growth factors and cytokines synthesized by the small and large luteal cells (del Canto et al., 2007, Niswender et al., 2000, Patton and Stouffer, 1991). Corpus luteum regression is independent of the uterus, and is probably due to reduced luteal LH sensitivity rather than systemic reductions in LH pulsatility (

The dynamic endometrium

The endometrium undergoes a steroid-induced monthly cycle of proliferation (regeneration during the follicular phase), differentiation (in the luteal phase) and shedding (menstruation). The upper functional layer is where the activities of proliferation, secretion and degeneration take place, and where the blastocyst implants following conception 7–10 days after ovulation (Strassmann, 1996); regeneration takes place in the lower basal layer (Jabbour et al., 2006). Oestradiol is responsible for

Process of menstruation

Cyclical growth and decline of the endometrium is universal in non-pregnant mammals, but external loss of blood is only seen in old world primates and very few other species (Martin, 2007). The luteal phase places metabolic demands on the woman, and it appears more economical to regenerate and shed than to continuously sustain a fully secretory endometrium (Strassmann, 1996). The cost of menstrual bleeding is considered low, and thus bleeding may only be a side-effect of a blood volume which

Conclusions

Reproduction in the woman contrasts markedly with economically important domestic animal species. High variability in cycle length and ovulation time within the cycle, low fertility per cycle, no overt sign of impending ovulation and the accelerated decline in fertility in our late thirties limit reproductive success. Similarities to the oestrous cycles of cows and mares proposed as model species for women exist and can potentially be exploited to address important questions, such as whether in

Conflict of interest

The authors have no conflict of interest in publishing this review.

References (72)

  • I.A. van Rooij et al.

    Serum antimullerian hormone levels best reflect the reproductive decline with age in normal women with proven fertility: a longitudinal study

    Fertil. Steril.

    (2005)
  • E.J.P. van Santbrink et al.

    Decremental follicle-stimulating-hormone and dominant follicle development during the normal menstrual-cycle

    Fertil. Steril.

    (1995)
  • L. Weller et al.

    Human menstrual synchrony: a critical assessment

    Neurosci. Biobehav. Rev.

    (1993)
  • J.L. Alcazar

    Three-dimensional ultrasound assessment of endometrial receptivity: a review

    Reprod. Biol. Endocrinol.

    (2006)
  • S. Araki et al.

    Biphasic stimulatory effects of estrogen on gonadotropin surges induced by continuous administration of gonadotropin-releasing hormone in women

    Endocrinol. Jpn.

    (1985)
  • S.E. Aydos et al.

    Is telomere length one of the determinants of reproductive life span?

    Arch. Gynecol. Obstet.

    (2005)
  • A.R. Baerwald et al.

    Endometrial development in association with ovarian follicular waves during the menstrual cycle

    Ultrasound Obstet. Gynecol.

    (2004)
  • A.R. Baerwald et al.

    Characterization of ovarian follicular wave dynamics in women

    Biol. Reprod.

    (2003)
  • A.R. Baerwald et al.

    Form and function of the corpus luteum during the human menstrual cycle

    Ultrasound Obstet. Gynecol.

    (2005)
  • O. Bakos et al.

    Ultrasonographical and hormonal description of the normal ovulatory menstrual cycle

    Acta Obstet. Gynecol. Scand.

    (1994)
  • Z.A.F. Cabral et al.

    Follicular growth pattern in normal-cycling Brazilian adolescents

    Fertil. Steril.

    (2007)
  • K. Chikazawa et al.

    Morphological and endocrinological studies on follicular development during the human menstrual cycle

    J. Clin. Endocrinol. Metab.

    (1986)
  • F. del Canto et al.

    Features of natural and gonadotropin-releasing hormone antagonist-induced corpus luteum regression and effects of in vivo human chorionic gonadotropin

    Clin. Endocrinol. Metab.

    (2007)
  • R. Ecochard et al.

    Side of ovulation and cycle characteristics in normally fertile women

    Hum. Reprod.

    (2000)
  • R. Fanchin et al.

    High reproducibility of serum anti-Müllerian hormone measurements suggests a multi-staged follicular secretion and strengthens its role in the assessment of ovarian follicular status

    Hum. Reprod.

    (2005)
  • H.M. Fraser et al.

    Vascular morphogenesis in the primate ovary

    Angiogenesis

    (2005)
  • M.A. Fritz et al.

    Onset and characteristics of the midcycle surge in bioactive and immunoactive luteinizing hormone secretion in normal women: influence of physiological variations in periovulatory ovarian steroid hormone secretion

    J. Clin. Endocrinol. Metab.

    (1992)
  • S.W. Gangestad et al.

    Human oestrus

    Proc. Biol. Sci.

    (2008)
  • O.J. Ginther et al.

    Follicle selection in monovular species

    Biol. Reprod.

    (2001)
  • O.J. Ginther et al.

    Comparative study of the dynamics of follicular waves in mares and women

    Biol. Reprod.

    (2004)
  • O.J. Ginther et al.

    Systemic concentrations of hormones during the development of follicular waves in mares and women: a comparative study

    Reproduction

    (2005)
  • N.H. Golden et al.

    The pathophysiology of amenorrhea in the adolescent

    Ann. N. Y. Acad. Sci.

    (2008)
  • R.R. Greb et al.

    A common single nucleotide polymorphism in exon 10 of the human follicle stimulating hormone receptor is a major determinant of length and hormonal dynamics of the menstrual cycle

    J. Clin. Endocrinol. Metab.

    (2005)
  • J.E. Hall et al.

    Hypothalamic gonadotropin-releasing hormone secretion and follicle-stimulating hormone dynamics during the luteal–follicular transition

    J. Clin. Endocrinol. Metab.

    (1992)
  • C. Hoekstra et al.

    Dizygotic twinning

    Hum. Reprod. Update

    (2008)
  • J.D. Hoff et al.

    Hormonal dynamics at midcycle: a reevaluation

    J. Clin. Endocrinol. Metab.

    (1983)
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    This paper is part of the special issue entitled: Reproductive Cycles of Animals, Guest Edited by Michael G. Diskin and Alexander Evans.

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    Present address: Department of Obstetrics and Gynaecology, ANU Research Centre, University College Cork, Cork, Ireland.

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