Pregnancy involves dramatic hormonal changes that support the growing fetus, maintain the uterine environment, and prepare the mother's body for birth and breastfeeding.

hCG (Human Chorionic Gonadotropin):

• Produced by the trophoblast (early placenta) starting at implantation
• Maintains the corpus luteum, which produces progesterone until the placenta takes over (~week 10)
• Causes nausea (morning sickness) in early pregnancy
• Levels peak around 8-11 weeks, then decline

Progesterone:

• Initially from corpus luteum, then from placenta
• Maintains the uterine lining (prevents menstruation)
• Suppresses uterine contractions (prevents early labor)
• Helps develop breast tissue for lactation

Estrogen:

• Stimulates uterine growth and development
• Increases blood flow to the uterus
• Promotes breast development
• Helps prepare the cervix for labor

Relaxin:

• Produced by corpus luteum and placenta
• Loosens pelvic ligaments (prepares birth canal)
• Softens the cervix
• Helps relax uterine smooth muscle

Human Placental Lactogen (hPL):

• Makes mother's body use more fat for energy
• Leaves more glucose available for the fetus
• Has growth hormone-like effects

What triggers labor? It's not fully understood, but we know several factors contribute to the process. The fetus actually plays an active role in signaling that it's ready to be born!

Fetal Signals:

• Fetal cortisol: Near term, the fetal adrenal glands produce increasing cortisol
• Fetal ACTH: The fetal pituitary releases ACTH, stimulating fetal adrenal activity
• These signals contribute to the hormonal cascade that initiates labor

Placental Changes:

• CRH (Corticotropin-Releasing Hormone): The placenta produces increasing amounts of CRH as pregnancy progresses
• High CRH levels correlate with labor onset

Hormonal Shift:

• Progesterone withdrawal: In many species, labor begins when progesterone drops. In humans, it's more complex — progesterone doesn't drop dramatically, but the uterus becomes less responsive to it
• Estrogen increase: Estrogen promotes oxytocin receptors on the uterus

Prostaglandins:

• Fetal membranes produce prostaglandins (especially PGF2α and PGE2)
• These stimulate uterine contractions and help soften/dilate the cervix
• Seminal fluid contains prostaglandins — one reason intercourse near term may help induce labor

Oxytocin is the key hormone of labor. Released by the posterior pituitary, it causes powerful uterine contractions. Importantly, oxytocin release creates a positive feedback loop — contractions stimulate more oxytocin, which causes stronger contractions.

Stage 1: Dilation (Longest Stage)

The cervix must dilate from closed (0 cm) to fully open (10 cm) to allow the baby to pass. This stage has three phases:

• Early labor (0-6 cm): Contractions 5-20 minutes apart, mild to moderate. Can last hours to days.
• Active labor (6-8 cm): Contractions 3-5 minutes apart, stronger and longer. cervix dilates more rapidly.
• Transition (8-10 cm): Most intense phase. Contractions 2-3 minutes apart, lasting 60-90 seconds. Often accompanied by nausea, trembling, and irritability.

Stage 2: Expulsion (Pushing and Birth)

Once fully dilated, the mother pushes with contractions:

• Baby descends through the birth canal
• Crowning: The baby's head becomes visible at the vaginal opening
• The head delivers first, then shoulders (the baby rotates to allow shoulders to pass)
• Rest of the body follows quickly
• The umbilical cord is clamped and cut

Stage 3: Placental Delivery

After the baby is born, the uterus continues to contract:

• The placenta separates from the uterine wall
• The placenta and membranes are expelled (usually within 5-30 minutes)
• Continued contractions compress blood vessels to prevent hemorrhage

After birth, dramatic hormonal changes occur as the mother's body transitions from pregnancy to lactation.

Hormonal Shifts:

• Estrogen and progesterone plummet: With the placenta gone, levels of these hormones drop dramatically
• Prolactin rises: Now free from progesterone's inhibitory effect, prolactin stimulates milk production
• Oxytocin: Continues to cause uterine contractions (helps the uterus shrink back to size) and triggers milk ejection

Lactation:

• Prolactin: Stimulates milk synthesis in the mammary glands
• Oxytocin: Causes myoepithelial cells to contract, ejecting milk (the "let-down reflex")
• Suckling stimulates both hormones — a positive feedback loop maintains milk production
• Colostrum (first milk) is rich in antibodies and immune cells

Uterine Involution:

• The uterus shrinks from ~1 kg to ~50-100 g over about 6 weeks
• Contractions (after-pains) help compress blood vessels and reduce size
• Breastfeeding stimulates oxytocin, which helps the uterus contract

Clinical Note: Postpartum hemorrhage (excessive bleeding) is a major concern. The uterus must contract firmly to compress the blood vessels where the placenta was attached. Oxytocin is often given after delivery to ensure strong contractions.