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HBIO4 > Contraception
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Menstrual Cycle

Follicular stage [days 1-13]

  • Pituitary gland secretes follicle-stimulating hormone (FSH)
  • FSH travels in blood to ovary
  • Stimulates development (division) of follicle cells surrounding the oocyte
  • Developing follicle cells secrete oestrogen
    • Stimulates proliferation of endometrium and its blood supply
    • Inhibits further secretion of FSH by negative feedback
    • Stimulates pituitary gland to secrete luteinising hormone (LH)
  • LH causes ovulation

Ovulation [day 14]

  • Mature ovarian follicle bursts and releases a 2° oocyte (NOT an ovum)
    • Happens once a month, menstrual cycle ≈28days
    • Each ovary alternatively releases a 2° oocyte every ≈56days (→56/2)
  • LH forms the corpus luteum from ruptured follicle

Luteal phase [days 15-28]

  • Corpus luteum secretes progesterone
  • Progesterone is responsible for
    • Proliferation of endometrium and its blood supply
    • Development of nutrient fluid glands in uterus lining
    • Inhibition of LSH and LH
  • Corpus luteum degenerates (due to inhibition of LH)

Menstruation [days 1-5]

  • Also called menses, period, menstrual bleeding
  • Sign that not pregnant
  • Shedding of the endometrium due to low levels of progesterone and oestrogen
  • FSH is not inhibited anymore → cycle starts again

Hormones

Maintaining Pregnancy

  • Human Chorionic Gonadotrophin (hCG)
    • Secreted by trophoblast and developing placenta
    • Prevents disintegration of corpus luteum (for 3mo)
    • Thereby, maintains high levels of progesterone
    • Basis for pregnancy test → hCG can be detected in urine
    • Peak in bloodstream after 2mo followed by a slow decline
  • Progesterone
    • Secreted by
      • Corpus luteum for the first 3mo
      • Placenta after 3mo (because the corpus luteum degenerates)
    • Prevents menstruation and maintains endometrium
    • Thickens cervical mucus → impermeable to sperm
    • Prepares uterus for implantation

Initiating Labour

  • Remember: oestrogen ↑contraction while progesterone ↓contraction of uterus
  • During labour, oestrogen:progesterone ratio increases
    • Progesterone falls
    • Oestrogen rises
      • Sensitises the uterus (muscle) to oxytocin
      • Releases relaxin which softens the pelvis
  • Pressure of fetus against cervix stimulates stretch receptors by positive feedback
    • Hypothalamus stimulates pituitary gland to secrete oxytocin
    • Oxytocin causes contraction of uterus
    • The higher the pressure, the more oxytocin released, the stronger the contractions

Milk production

  • Progesterone inhibits prolactin
  • At birth, progesterone levels fall → prolactin levels increase
  • Breastfeeding promotes production of milk by positive feedback
    • Nerve impulses travel to hypothalamus
    • Stimulates pituitary gland to secrete oxytocin and prolactin
    • The more milk is removed, the more hormones are secreted
  • Oxytocin
    • Stimulates contraction of milk ducts → squeezes milk out
  • Prolactin
    • Stimulates milk production
    • Inhibits secretion of FSH and LH → prevents ovulation → less likely to conceive

Contraception

Hormones

  • 99% effective (<1:100 women will get pregnant in a year)
  • Progestogen-only pill
    • Thickens cervical mucus → reduces sperm motility
    • Inhibits LH production → may prevent ovulation (depends on dose)
  • Combined pill (oestrogen + progestogen)
    • Oestrogen inhibits FSH production → prevents follicular development
    • Taken for 21 days - withdrawal bleed for the last 7 days
    • Reduce risk of ovarian and endometrial cancer
    • Higher risk of thrombosis in smokers
  • Long-acting progestogen can be given for up to 3mo by injection
  • Morning-after pill
    • Contains high doses of progestogen
    • Works for up to 72h after intercourse - works best within 12h

IUD

  • 99% effective
  • Small plastic and copper device
  • Inserted into uterus for up to 10yrs
  • Mechanism
    • Stimulates release of prostaglandins by endometrium
    • Copper kills sperm
  • Risk of ectopic pregnancy (ovulation not inhibited) and infection
  • May cause heavy periods

Barrier

  • Condoms
    • Male condom - 98% effective and protect from STDs
    • Female condom - 95% effective and may protect from STDs
  • Cap with spermicide
    • 92-6% effective
    • Latex silicon cap is used with spermicide, is put into vagina to cover cervix
    • Must be specifically fitted to make sure it is the right size
    • May protect against some STDs and cancer of the cervix
    • After intercourse, it should be left in place for 6-8h
    • Not used if a woman had toxic shock syndrome (infection due to tampon)

Causes of Infertility

  • Low sperm count
    • Varicocele → dilated veins in scrotum
    • Blockage of vasa deferentia
    • Sexually transmitted infections (STDs)
    • Hormone dysfunction
    • Diabetes
  • Blocked oviducts
    • STDs

In vitro fertilisation (IVF)

  • FSH is injected in woman to stimulate development of several follicles
  • Just before ovulation, oocytes are collected from the ovary and matured
  • Fertilisation
    • Collected sperm is added to the collected oocyte
    • Nucleus of sperm is micro-injected into the oocyte (intra-cytoplasmic sperm injection)
    • Fertilisation outside the body in a flat glass dish
  • Culture to the 8-16 cell stage
  • Maximum of three are transferred back into the uterus
  • Advantage: possible to screen embryos for genetic defects LINK WITH 3-4-2