• Shortly after the developing embryo implants into the endometrium it begins secretion of the hormone?human chorionic gonadotropin?(hCG)
• hCG is secreted during the first?8-10 weeks?of pregnancy
• The role of this hormone is to:
  • Stimulate the corpus luteum?in the ovary to?maintain secretion of oestrogen and progesterone?(in order to continue the development of the endometrium)
  • Stimulate?the?growth of the placenta?and?uterine enlargement
  • Inhibit menstruation
• During the second trimester (after?12 weeks) hCG?declines?and the placenta takes over the role of stimulating the secretion of oestrogen and progesterone
• Pregnancy tests detect the presence of hCG in the mother's urine and can be used to confirm a positive pregnancy

hCG increases during the first 12 weeks of pregnancy to stimulate the release of oestrogen and progesterone; after 12 weeks hCG declines as the placenta takes over

The role of progesterone

• The hormone progesterone is secreted by the placenta throughout pregnancy
• Progesterone?inhibits the production?of another hormone,?oxytocin, by the?pituitary gland
• Oxytocin?inhibits contractions?of the?muscles?of the uterus wall (the?myometrium), which could induce birth if not inhibited
• At the?end of pregnancy, the?foetus?produces the hormone?oestrogen, which signals to the placenta to?stop producing progesterone, thereby?initiating the production of oxytocin?by the pituitary gland and the start of labour (and the start of the muscular contractions that eventually lead to the birth of the baby)

The role of oestrogen

• At the end of pregnancy, the hormone?oestrogen?is produced by the?foetus?and the?placenta
• Oestrogen makes the?uterine wall more sensitive to oxytocin
• Progesterone?is also?inhibited?by oestrogen

Labour

• Oxytocin now stimulates?contractions?of the muscles in the?myometrium
  • Oxytocin is released by the pituitary gland in the brain
• Stretch receptors?in the?cervix?detect the contractions and signal the pituitary gland to?increase oxytocin secretion
• More oxytocin creates?further contractions, which in turn signal for?further release of oxytocin?in this?positive feedback loop
• This process increases the contractions slowly and rhythmically

The positive feedback loop stimulates the release of oxytocin and causes the contraction of the uterus wall

Birth

• Relaxation?of the?cervix muscles?causes the cervix to?dilate?and?widen
• Uterine contractions continue and cause the?amniotic sac to burst, releasing the amniotic fluid through the open cervix
• The baby is?pushed out?through the?cervix?and?vagina?as contractions continue
• As the umbilical cord is cut, the baby is?physiologically separated?from the mother

Afterbirth

• After the baby has been delivered,?uterine contractions continue?and the placenta will separate from the uterine wall
• The mother will then?birth the placenta?and remains of the umbilical cord

The stages of birth, which are initiated by the hormone oxytocin