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Phases of the Menstrual Cycle

There are actually four phases in the menstrual cycle, although it’s usually divided into two phases of about 14 days each, with the period and ovulation marking the beginnings and ends of them:

  1. The “follicular phase” starts with the period and ends at ovulation
  2. The “luteal phase” begins at ovulation and ends with the period

Figure showing changing hormone levels, follicle development and temperature across the menstrual cycle

However, there are FOUR parts to a menstrual cycle, and we use a colour code to help to show how it works (eggs and follicles may help too):

  • The hypothalamus hormone is red: gonadotropin release hormone (GnRH)
  • Hormones from the anterior pituitary are green: follicle-stimulating hormone (FSH) and luteinising hormone (LH)
  • Ovarian hormones blue: estrogen and progesterone
  • The placenta hormone is orange: human chorionic gonadotropin (hCG)

Day 1-7 of the cycle

  • Progesterone levels from the corpus luteum have dropped, and the hypothalamus is no longer suppressed from releasing GnRH, which triggers the start of a new cycle
  • Low progesterone also causes the womb lining’s upper ‘functional’ layer to detach, and the period begins
  • GnRH stimulates the anterior pituitary gland to produce FSH & LH
  • FSH stimulates follicles in the ovaries to develop
  • LH stimulates the larger follicles in the ovaries to produce estrogens
  • Rising estrogen levels stimulate a new functional layer in the uterus to grow, even as the old one is lost in the period

Days 7-14 of the cycle

  • FSH stimulates the development of the large mature follicles, and one matures into a dominant follicle
  • All large follicles produce estrogen, which stimulates the growth of the womb lining
  • Rising estrogen level triggers a sudden surge in LH and FSH (usually around day 13)
  • The LH surge triggers the final maturation of the dominant follicle; the egg detaches from the follicle wall and is released at ovulation 24-36 hours later

Day 14-21 of the cycle

  • After ovulation, the released egg is collected by one of the Fallopian tubes, and fertilisation is in the tube
  • The LH surge has already started the transformation of the ruptured dominant follicle into a corpus luteum. This now fills with blood, and it starts to produce progesterone
  • The womb lining responds to progesterone, becoming more spongy and “sticky” to help the chances of implantation

Days 21-28 of the cycle

  • A high progesterone level encourages changes in the endometrial lining to maximise the chances of implantation
  • If an embryo implants, the placenta releases hCG, which stimulates the corpus luteum to increase progesterone and prevent the start of a new cycle
  • Without implantation, the corpus luteum degenerates, and progesterone and estrogen levels fall. This means the hypothalamus is no longer prevented from producing GnRH, and a new cycle is triggered

Estrogen and progesterone explain the importance of these hormones in conception

Cycle regularity and fertility

Most menstrual cycles aren’t “like clockwork”, and only about 3% of women have cycle length variation of fewer than three days in a year. Nearly half (42%) of women have cycles that vary by seven days a year. i This is important as cycle regularity has a big effect on fertility:

  • Variation in cycle length strongly predicts monthly pregnancy rates ii
  • Cycle regularity affects pregnancy rates more than cycle length
  • Cycle lengths that vary by over ten days a year have about 25% of normal pregnancy rates
  • Very irregular cycles indicate poor hormone stability
  • When cycles become more regular, the chances of pregnancy increase

Menstrual cycle temperatures

The menstrual cycle is dominated by estrogens in the follicular phase and progesterone in the luteal phase. These hormones affect the basal body temperature and produce a pattern of two distinct (BBT) temperatures in normal cycles:

      1. The follicular phase is 36.2 – 36.5°C (if below 36.0°C low thyroid hormones could be a problem)
      2. The luteal phase is about 36.6 – 37.0°C

      1. Healthy cycles have:
          • A temperature rises from the follicular to the luteal phase (after ovulation) of 0.4–0.5°C over 1-2 days
          • A temperature falls just before the period by 0.4-0.5°C over 1–2 days
          • Follicular phases are 12-16 days (10–20 days are the extremes of normal)
          • Luteal phases are 12-16 days (over 18 days indicates a pregnancy)

        i Natürliche Familienplanung heute: Modernes Zykluswissen für Beratung.  By Elisabeth Raith-Paula, Petra Frank-Herrmann, Günter Freundl, Thomas Strowitzki. Springer Press. 2008
        ii ‘Menstrual cycle pattern and fertility: a prospective follow-up study of pregnancy and early embryonal loss in 295 couples who were planning their first pregnancy’. Henrik A. Kolstad, et al. Fertility and Sterility Volume 71, Issue 3, Pages 490-496, March 1999
        By Kimanh Nguyen – https://mla-lima.wikispaces.com/Anatomy+of+the+Ovaries? responseToken=01fcd85f07c85da7489dd7e2a01e82bf0, CC BY-SA 3.0, Link
        Illustration from Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.

        Photo by Celso on Unsplash