BIO 304 · Human Anatomy & Physiology
Pregnancy A&P (Basics)
Reproductive System · Module 16
A reference for the Pregnancy A&P video. From a single fertilized egg to a delivered newborn is ~40 weeks. Three trimesters, three different physiologic chapters. Almost every organ system in the mother adapts.
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- Trace fertilization through implantation and identify the role of the placenta.
- List the major maternal physiological changes by system.
- Describe the three stages of labor and how hormones drive each.
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Fertilization, Implantation, Placenta
Fertilization
- Siteusually ampulla of fallopian tube
- Timingwithin 24 hours of ovulation
- Acrosome reactionsperm enzymes penetrate zona pellucida
- Cortical reactionfast block to polyspermy after first sperm enters
- Zygotefertilized single cell; diploid
First week (in transit)
- Cleavagemitotic divisions without growth
- Morula16-cell solid ball; ~day 3
- Blastocysthollow ball; inner cell mass + trophoblast; ~day 5
- Hatching from zona pellucida~day 6
- Implantation~day 7; trophoblast invades endometrium
Placenta
- Formed bymaternal endometrium + fetal trophoblast (chorionic villi)
- Functiongas, nutrient, waste exchange; hormone production; barrier
- Maternal blood lakesaround chorionic villi; no direct mixing of blood
- Umbilical cordtwo arteries (carry deoxygenated blood to placenta) + one vein (oxygenated to fetus)
Placental hormones
- hCGfrom trophoblast; maintains corpus luteum until placenta takes over
- Estrogenmaternal effects, uterine growth, breast development
- Progesteronemaintains pregnancy; suppresses contractions
- hPL (human placental lactogen)maternal insulin resistance, glucose available for fetus
- Relaxinsoftens cervix; relaxes pelvic ligaments
Maternal Changes & Labor
Trimesters
- First trimester (0-12 wks)organogenesis; most vulnerable to teratogens; hCG peaks
- Second trimester (13-27 wks)fetal growth, movement felt; least symptomatic for mother
- Third trimester (28-40 wks)rapid growth; mother's adaptations maximal
Cardiovascular changes
- Blood volumeup 40-50%
- Cardiac outputup 30-50%
- HRup 10-20 bpm
- BPfalls in T2 (vasodilation) then returns
- Plasma greater than RBC massphysiologic anemia of pregnancy
Respiratory changes
- Tidal volumeup; minute ventilation up ~50%
- PaCO₂drops to ~30; mild respiratory alkalosis (renal compensates)
- Diaphragmdisplaced upward by uterus in late pregnancy
Renal & GI changes
- GFRup ~50%
- Refluxincreased due to LES relaxation (progesterone) + uterine pressure
- Constipationprogesterone slows motility
- Glucose toleranceincreased insulin resistance (hPL); gestational diabetes risk
Stages of labor
- Stage 1 (longest)cervical effacement and dilation; ends at full dilation (10 cm)
- Stage 2expulsion of baby; ends at delivery
- Stage 3 (shortest)delivery of placenta; usually within 30 min
- Stage 4 (some define)immediate postpartum recovery; risk of hemorrhage
Hormones of labor
- Oxytocinuterine contractions; positive feedback amplifies; nipple stimulation also triggers
- Prostaglandinssoften cervix; stimulate contractions
- Relaxinlate pregnancy; cervical and pelvic preparation
- CRH (placental)rises near term; influences timing of labor
Lactation
- Prolactinmilk production after delivery (estrogen/progesterone fall is the trigger)
- Oxytocinmilk ejection (let-down reflex)
- Colostrumfirst milk; rich in IgA & immune factors
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