Changes During Pregnancy, Labor, and Birth

Core Concepts

The placenta as an endocrine organ

The placenta does far more than exchange gases and nutrients — it is a hormone factory. Early on, its trophoblast cells secrete hCG, which rescues the corpus luteum so it keeps producing progesterone until the placenta matures. Around weeks 7–12 (the commonly taught "luteoplacental shift"), the placenta becomes the main source of progesterone and estrogen. It also makes human placental lactogen (hPL), which redirects maternal metabolism toward the fetus, and relaxin, which softens pelvic connective tissue and the cervix.

Maternal adaptations by organ system

• Cardiovascular: Blood volume expands by roughly 30–50% (a commonly taught reference) and cardiac output rises; lying supine lets the heavy uterus compress the vena cava, reducing venous return — why side-lying rest is advised.
• Respiratory: Progesterone raises the brain's sensitivity to carbon dioxide, so breathing deepens (higher tidal volume) and the mother may feel breathless at a normal rate; the uterus also pushes the diaphragm upward.
• Urinary: Glomerular filtration rate rises, urine output increases, and the compressed bladder causes frequent urination.
• Gastrointestinal: Progesterone relaxes smooth muscle, slowing gut motility (constipation) and relaxing the lower esophageal sphincter (heartburn). Early nausea — "morning sickness" — is commonly attributed to rising hCG and estrogen.
• Musculoskeletal: Relaxin loosens pelvic ligaments and widens the pubic symphysis; the growing uterus shifts the center of gravity forward, straining the lower back.

The uterus and fetus before labor

The uterus grows from roughly pear-sized to occupying most of the abdomen. In the third trimester many pregnant people feel Braxton Hicks contractions — irregular, usually painless tightenings with no cervical change; these are "practice" contractions, not true labor. Near term the fetus usually settles head-down (cephalic presentation); breech presentation is less common and may influence how birth is managed.

Labor: three stages

Labor uses rhythmic uterine contractions to dilate the cervix and expel the fetus and placenta.

• Stage 1 — Dilation: The cervix thins (effacement) and opens to a full 10 cm, subdivided into latent, active, and transition phases as contractions become regular, longer, and stronger.
• Stage 2 — Expulsion: At full dilation, the birthing person pushes and the fetus descends through the pelvis and vagina; crowning occurs when the head is visible at the vaginal opening.
• Stage 3 — Placental delivery: Continued contractions shear the placenta from the uterine wall. Clinicians check that it is complete, because retained fragments are a recognized risk.

Hormonal control of labor

The engine of labor is a positive-feedback loop built on oxytocin. Cervical and vaginal stretch sends nerve signals to the hypothalamus, triggering posterior-pituitary oxytocin release; oxytocin strengthens contractions; stronger contractions stretch the cervix further, releasing more oxytocin — the cascade builds until birth. Near term, progesterone's quieting effect declines relative to estrogen, and prostaglandins sensitize the uterus. Synthetic oxytocin is used clinically to start or strengthen labor, but protocols vary by institution and are outside this guide.

True labor versus false labor

True labor contractions are regular, grow stronger and closer together, continue despite activity, and produce progressive cervical change. False labor (Braxton Hicks) contractions are irregular, do not intensify, often stop with rest or position change, and cause no cervical change. Rupture of the amniotic membranes ("water breaking") and passage of the mucus plug ("bloody show") are additional signs that labor is near.

Common Confusions