Pregnancy in humans lasts approximately 266 days (38 weeks) from fertilization, or 40 weeks from the last menstrual period (LMP). The placenta serves as the vital structural and functional link between the mother and fetus throughout this period.

The placenta performs four primary functions: nutrient and gas exchange (oxygen delivery and $CO_{2}$ removal between maternal and fetal circulations), waste removal (urea, bilirubin transferred from fetus to maternal blood), and hormone secretion. The hormones produced by the placenta include hCG (human chorionic gonadotropin, which maintains the corpus luteum during early pregnancy), hPL (human placental lactogen, which prepares mammary glands for lactation and increases maternal blood glucose for fetal nutrition), estrogen, and progesterone. Around weeks 8–10 of pregnancy, the placenta assumes primary responsibility for progesterone production from the corpus luteum — this is called the luteal-placental shift.

Parturition (childbirth) begins with signals from the fully developed fetus. Fetal cortisol triggers a shift in placental steroid biosynthesis from progesterone dominance to estrogen dominance, increasing the sensitivity of the myometrium to oxytocin. Oxytocin, released from the posterior pituitary, stimulates uterine muscle contractions. Critically, parturition involves a positive feedback loop: uterine contractions cause cervical stretching → sensory signals to the hypothalamus → more oxytocin released → stronger contractions → more cervical stretch → yet more oxytocin. This escalating loop continues until delivery, at which point the feedback stimulus (cervical stretch) is removed and the loop breaks.

Following delivery, two hormones coordinate lactation. Prolactin (anterior pituitary) stimulates the synthesis and secretion of milk by mammary alveolar cells. Oxytocin (posterior pituitary), released in response to infant suckling, triggers the milk ejection (let-down) reflex by causing myoepithelial cells surrounding the mammary alveoli to contract, expelling milk toward the nipple. Although prolactin levels are elevated throughout pregnancy, high estrogen and progesterone competitively inhibit prolactin's action on mammary cells — meaning milk production begins only after delivery, when placental estrogen and progesterone levels fall dramatically.