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Human Physiology · Endocrine and reproduction

Reproduction, Pregnancy and Lactation

Reproduction is controlled by a hormonal axis from the brain to the gonads. Pregnancy then reshapes the heart, lungs, metabolism, joints and pelvic floor. This page explains the physiology and the evidence for exercise and pelvic floor training around childbirth.

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Part 8 · Endocrine and Reproduction · Chapter 39 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the hypothalamic-pituitary-gonadal axis in men and women.
  • Explain the hormonal events of the menstrual cycle.
  • Describe the main maternal adaptations to pregnancy.
  • Explain the hormonal control of lactation.
  • Summarise the evidence for prenatal exercise and pelvic floor training.
  • Adapt exercise safely in pregnancy and after birth.

Why it matters

Physiotherapists treat women before, during and after pregnancy for back and pelvic girdle pain, incontinence and deconditioning. They also see athletes whose periods stop from under-fuelling.

Knowing the physiology helps you prescribe exercise confidently rather than restricting women unnecessarily.

The reproductive axis

Figure 1 · The reproductive axis

Hypothalamic-pituitary-gonadal axis in men and women The hypothalamus releases GnRH in pulses, driving the anterior pituitary to release LH and FSH. In the testis these drive testosterone and sperm production; in the ovary follicle growth, oestrogen, ovulation and progesterone. Sex steroids feed back to the hypothalamus and pituitary, mostly negatively. Hypothalamus: GnRHreleased in pulsesAnterior pituitaryLH and FSHTestisLH: Leydig cells make testosteroneFSH + testosterone: spermOvaryFSH: follicle grows, oestrogenLH surge: ovulation, then progesteronenegativefeedbackmostly negative;positive beforeovulation
Energy matters. GnRH pulses are suppressed by low energy availability, heavy stress and illness, which is why periods can stop in under-fuelled athletes.
HormoneSourceMain actions
GnRHHypothalamusPulses drive LH and FSH release
LHAnterior pituitaryTestosterone from Leydig cells; ovulation and corpus luteum
FSHAnterior pituitarySperm production; follicle growth
TestosteroneTestisMale characteristics, muscle and bone mass, libido
OestrogenOvarian follicleFemale characteristics, endometrium growth, bone protection
ProgesteroneCorpus luteum, placentaPrepares and maintains the endometrium; raises body temperature

Testosterone and oestrogen both help maintain bone. Loss of either, through menopause, low energy availability or hormone treatment, accelerates bone loss.

The menstrual cycle

Figure 2 · Hormones of the menstrual cycle

Oestrogen, progesterone and LH levels across a 28-day cycle Oestrogen rises in the follicular phase and peaks just before a sharp LH surge that triggers ovulation near day 14. Progesterone dominates the luteal phase with a second smaller oestrogen rise, and both fall before menses. mensesovulation, about day 14follicular phaseluteal phase07142128Relative levelDay of cycleOestrogenProgesteroneLH
Rising oestrogen flips feedback to positive, producing the LH surge. Progesterone raises body temperature slightly after ovulation. Schematic.

In the follicular phase, FSH grows a dominant follicle that secretes rising oestrogen. High oestrogen switches feedback to positive and triggers the LH surge and ovulation.

The empty follicle becomes the corpus luteum, secreting progesterone and oestrogen. Without pregnancy it regresses after about 14 days, hormones fall and menstruation begins.

Low energy availability

Athletes who eat too little for their training can lose GnRH pulsatility. Periods become irregular or stop, oestrogen falls and bone is lost. Missed periods in an athlete need assessment, not reassurance.

Pregnancy

After fertilisation, the embryo implants and the placenta forms. Placental hCG maintains the corpus luteum until the placenta itself makes progesterone and oestrogen.

Figure 3 · How the mother adapts

Main physiological adaptations of pregnancy relevant to exercise Eight boxes: blood volume rises by about 40 to 50 per cent, cardiac output by about 30 to 50 per cent, ventilation rises causing mild respiratory alkalosis, insulin resistance increases in later pregnancy, ligaments become lax, the centre of mass moves forward, the pelvic floor is loaded and stretched, and lying supine late in pregnancy can compress the vena cava. Blood volumeup about 40–50%Cardiac outputup about 30–50%Ventilationup; mild respiratory alkalosisInsulin resistancerises in later pregnancyLigament laxityrelaxin and oestrogenPosturecentre of mass moves forwardPelvic floorload and stretch at birthLying supine late onuterus can compress vena cava
Plan exercise around these changes. Most healthy pregnant women can exercise safely with sensible adjustments. Typical textbook values.

Rising blood volume, cardiac output and ventilation support the fetus. Later in pregnancy, placental hormones increase insulin resistance, which unmasks gestational diabetes in susceptible women.

Davenport and colleagues pooled 106 studies with 273,182 participants [1]. Exercise-only interventions reduced the odds of gestational diabetes, gestational hypertension and pre-eclampsia [1].

Figure 4 · Exercise protects against pregnancy complications

Odds ratios for gestational diabetes, gestational hypertension and pre-eclampsia with prenatal exercise Exercise-only interventions reduced the odds of gestational diabetes to 0.62, gestational hypertension to 0.61 and pre-eclampsia to 0.59 compared with no exercise, with all confidence intervals below one. Gestational diabetesOR 0.62Gestational hypertensionOR 0.61Pre-eclampsiaOR 0.590.250.51Odds ratio, exercise only versus no exercise (left of 1 favours exercise)Benefit needed about 600 MET-min/week, e.g. 140 min brisk walking [1]
About 40% lower odds of each. Exercise combined with other interventions did not show the same benefit in this review [1].

Accumulating about 600 MET-minutes a week of moderate activity, roughly 140 minutes of brisk walking, gave at least a 25% reduction [1].

Birth and the pelvic floor

Oxytocin drives uterine contractions in a positive feedback loop during labour. Vaginal birth stretches and can injure the pelvic floor muscles, nerves and connective tissue.

About one third of women have urinary incontinence after childbirth [2].

Figure 5 · Pelvic floor training prevents leakage

Risk ratios for urinary incontinence with pelvic floor muscle training around pregnancy In continent pregnant women, antenatal pelvic floor training cut the risk of incontinence in late pregnancy by 62 per cent and at three to six months after birth by 29 per cent. In mixed groups the reduction was 22 per cent. Postnatal training for persistent leakage had an uncertain effect. Continent, antenatal PFMT: late pregnancy62% lessContinent, antenatal PFMT: 3–6 months after birth29% lessMixed group, antenatal PFMT: late pregnancy22% lessPersistent leakage, postnatal PFMT: 6–12 monthsuncertain0.250.51Risk ratio for urinary incontinence versus usual care46 trials, 10,832 women [2]
Start early, target continent women. Prevention was clearer than treatment in this population [2].

A Cochrane review of 46 trials and 10,832 women found that antenatal pelvic floor muscle training (PFMT) in continent women reduced incontinence in late pregnancy and the months after birth [2]. Effects as treatment were less certain [2].

Lactation

HormoneRole in breastfeeding
ProlactinStimulates milk production; rises with suckling
OxytocinMilk ejection (let-down); also contracts the uterus
Oestrogen and progesteronePrepare the breast in pregnancy; their fall after birth allows milk production

High prolactin during breastfeeding suppresses GnRH, so periods often return late. Low oestrogen during lactation also causes temporary bone loss that usually recovers.

Clinical relevance for the physiotherapist

  • Encourage activity. Moderate exercise lowers the odds of major pregnancy complications [1].
  • Teach PFMT early. Start in early pregnancy in continent women [2].
  • Late pregnancy. Avoid long periods lying flat if dizzy; use side-lying or inclined positions.
  • Laxity and posture. Strengthen hips and trunk for pelvic girdle comfort.
  • Athletes. Screen menstrual function and energy intake.

Assessment and application

SituationInterpretationAction
Healthy pregnant woman, inactiveExercise reduces complications [1]Build to about 140 min/week moderate activity
Continent woman at 12 weeksGood candidate for prevention [2]Structured PFMT programme
Dizziness lying supine at 34 weeksVena cava compressionTurn onto left side; adapt positions
Runner with no periods for six monthsPossible low energy availabilityRefer for medical and dietary assessment

Common mistakes

  • Telling healthy pregnant women to rest [1].
  • Leaving pelvic floor training until problems appear [2].
  • Assuming missed periods in athletes are harmless.
  • Ignoring warning signs such as bleeding, chest pain or reduced fetal movement.
  • Equating ligament laxity with instability needing rest.

Exam preparation

Long answer

Describe the hormonal regulation of the menstrual cycle. Add a note on physiological changes in pregnancy.

  • Hypothalamic-pituitary-ovarian axis (Figure 1).
  • Follicular and luteal phases, LH surge (Figure 2).
  • Placental hormones.
  • Cardiovascular, respiratory, metabolic and musculoskeletal changes (Figure 3).
  • Exercise and pelvic floor evidence [1, 2].

Short notes

  • LH surge.
  • Functions of progesterone.
  • Milk ejection reflex.
  • Pelvic floor muscle training in pregnancy [2].

Viva and practical

  • “What triggers ovulation?” — the LH surge.
  • “Which hormone causes milk let-down?” — oxytocin.
  • “Should pregnant women exercise?” — yes, about 140 min/week moderate [1].
  • OSCE: teach a pelvic floor contraction with correct breathing.

Current evidence

Exam and traditional teaching

Pregnancy is a time for caution and reduced activity.

What the evidence adds

Prenatal exercise lowers the odds of gestational diabetes, hypertension and pre-eclampsia [1].

Early antenatal PFMT in continent women helps prevent incontinence [2].

What the student should understand

Pregnancy is a normal physiological state. Active women with a trained pelvic floor tend to have fewer problems.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. Exercise-only interventions in pregnancy reduced the odds of gestational diabetes to about:
  1. (A) 0.12
  2. (B) 0.62
  3. (C) 1.0
  4. (D) 1.6

Answer: (B) The odds ratio was 0.62 (0.52 to 0.75) [1].

Q2. Which other complications did prenatal exercise reduce?
  1. (A) None
  2. (B) Gestational hypertension and pre-eclampsia
  3. (C) Only preterm birth
  4. (D) Only back pain

Answer: (B) Odds ratios were 0.61 for gestational hypertension and 0.59 for pre-eclampsia [1].

Q3. To reduce the odds of these complications by at least 25%, women needed about:
  1. (A) 60 MET-min per week
  2. (B) 600 MET-min per week of moderate exercise
  3. (C) 6,000 MET-min per week
  4. (D) Any amount

Answer: (B) About 600 MET-min/week, such as 140 minutes of brisk walking [1].

Q4. Exercise combined with other interventions such as diet:
  1. (A) Worked better than exercise alone
  2. (B) Did not show the same reduction in this review
  3. (C) Was harmful
  4. (D) Was not studied

Answer: (B) Exercise-only, but not exercise plus co-interventions, reduced the odds [1].

Q5. Which of these counts toward the recommended prenatal activity?
  1. (A) Brisk walking, water aerobics, stationary cycling or resistance training
  2. (B) Only swimming
  3. (C) Only yoga
  4. (D) Nothing; rest is advised

Answer: (A) All were given as examples of moderate-intensity exercise [1].

Q6. In continent pregnant women, antenatal PFMT reduced urinary incontinence in late pregnancy by about:
  1. (A) 5%
  2. (B) 22%
  3. (C) 62%
  4. (D) 100%

Answer: (C) Risk ratio 0.38, a 62% reduction [2].

Q7. At three to six months after birth, antenatal PFMT in continent women reduced incontinence by about:
  1. (A) 29%
  2. (B) 62%
  3. (C) 90%
  4. (D) It increased it

Answer: (A) Risk ratio 0.71, a 29% reduction [2].

Q8. For women with persistent incontinence after birth, the effect of postnatal PFMT at 6–12 months was:
  1. (A) Clearly large
  2. (B) Uncertain
  3. (C) Harmful
  4. (D) Not measured

Answer: (B) Risk ratio 0.55 with an interval crossing one; low-quality evidence [2].

Q9. About what proportion of women have urinary incontinence after childbirth?
  1. (A) One in a hundred
  2. (B) About one third
  3. (C) Almost all
  4. (D) None

Answer: (B) About one third of women have urinary incontinence after childbirth [2].

Q10. The Cochrane review concluded that the clearest benefit of PFMT is:
  1. (A) Treatment of established leakage after birth
  2. (B) Early, structured antenatal training in continent women to prevent leakage
  3. (C) Faecal incontinence cure
  4. (D) No benefit

Answer: (B) Early structured antenatal PFMT in continent women may prevent incontinence [2].

Quick review

Key takeaways

  • GnRH pulses drive LH and FSH; sex steroids feed back.
  • Oestrogen peak triggers the LH surge and ovulation.
  • Pregnancy raises blood volume, cardiac output, ventilation and insulin resistance.
  • Prenatal exercise lowers odds of GDM, hypertension and pre-eclampsia by about 40% [1].
  • Antenatal PFMT in continent women cut late-pregnancy leakage by 62% [2].
  • Prolactin makes milk; oxytocin ejects it.

Further reading

SourceWhy it earns its place
Davenport MH et al. — Br J Sports Med, 2018Prenatal exercise and complications [1]
Woodley SJ et al. — Cochrane, 2020Pelvic floor training around childbirth [2]
Guyton and Hall, Textbook of Medical PhysiologyReproductive physiology chapters

References

  1. Davenport MH, Ruchat SM, Poitras VJ, et al. Prenatal exercise for the prevention of gestational diabetes mellitus and hypertensive disorders of pregnancy: a systematic review and meta-analysis. British Journal of Sports Medicine. 2018;52(21):1367–1375. doi:10.1136/bjsports-2018-099355 PMID 30337463 Systematic review and meta-analysis
  2. Woodley SJ, Lawrenson P, Boyle R, et al. Pelvic floor muscle training for preventing and treating urinary and faecal incontinence in antenatal and postnatal women. Cochrane Database of Systematic Reviews. 2020;5(5):CD007471. doi:10.1002/14651858.CD007471.pub4 PMID 32378735 Systematic review and meta-analysis

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Reviewed by the Physiotherapist India Team. · Human Physiology contents