Pregnancy
Physiology Overview
Explore the remarkable adaptations of the human body during pregnancy: from fertilization to systemic maternal changes that sustain new life.
Anatomy & Physiology
What is Pregnancy Physiology?
Pregnancy physiology is the study of how the maternal body adapts at every organ-system level to support the growth and development of a fetus. These adaptations begin at fertilization and continue through parturition (birth) and into the postpartum period.
Essential Vocabulary
Fertilization
Union of sperm and oocyte, forming a diploid zygote \(2n = 46\)
Implantation
Blastocyst embeds in the uterine endometrium, ~6-10 days post-fertilization
Placenta
Organ of exchange for \(\text{O}_2\), nutrients, and waste between mother and fetus
hCG
Human chorionic gonadotropin: hormone that maintains the corpus luteum early in pregnancy
Parturition
The process of labor and delivery driven by oxytocin positive feedback
Fertilization to Implantation
The Hormonal Cascade
Placental Functions
Trimester Comparison
| 1st Trimester | 2nd Trimester | 3rd Trimester | |
|---|---|---|---|
| Weeks | 1-12 | 13-26 | 27-40 |
| Key Hormone | hCG peak | Placental hormones | Oxytocin, relaxin |
| Fetal Focus | Organogenesis | Growth & maturation | Lung maturity, weight |
| Maternal Change | Morning sickness | Blood volume ↑ 40-50% | Cardiac output max |
Cardiovascular Adaptations
Respiratory & Renal Changes
Tidal Volume
Increases ~40% to meet elevated \(\text{O}_2\) demand; driven by progesterone stimulating the respiratory center
Minute Ventilation
Rises ~50% (tidal volume up, rate slightly up), causing a mild respiratory alkalosis
GFR
Glomerular filtration rate increases ~50% due to higher blood volume and renal plasma flow
Kidney Size
Kidneys enlarge ~1 cm; ureters dilate (especially right side) due to progesterone and uterine compression
Endocrine Shifts in Pregnancy
1
hCG: peaks at ~10 weeks, maintains corpus luteum, basis of pregnancy tests
2
Progesterone: relaxes smooth muscle (uterine quiescence), promotes endometrial support
3
Estrogen: stimulates uterine growth, mammary duct development, increases uterine blood flow
4
hPL (human placental lactogen): mobilizes maternal glucose and fatty acids for fetal nutrition
5
Relaxin: softens the cervix and loosens pelvic ligaments in preparation for delivery
Embryo vs. Fetus
| Embryo | Fetus | |
|---|---|---|
| Time Period | Weeks 3-8 | Week 9 to birth |
| Primary Process | Organogenesis | Growth & maturation |
| Vulnerability | Highest teratogen risk | Lower, but still present |
| Size (end) | ~3 cm, ~1 g | ~50 cm, ~3.4 kg |
| Key Event | Neural tube closure | Lung surfactant production |
How the Body Prepares for Labor
1
Estrogen rises relative to progesterone, increasing uterine sensitivity to oxytocin
2
Oxytocin receptors multiply on uterine myometrium (up to 200-fold increase)
3
Fetal head descends and presses on the cervix, triggering stretch receptors
4
Positive feedback loop: cervical stretch → oxytocin release → stronger contractions → more stretch
5
Prostaglandins soften and thin the cervix (effacement and dilation)
Positive Feedback in Parturition
Worked Example: Blood Volume
A non-pregnant woman has a blood volume of 5 L. During pregnancy, blood volume increases by approximately 45%. What is her expected blood volume at peak pregnancy?
1
Identify values: baseline = \(5\,\text{L}\), increase = \(45\%\)
2
Calculate increase: \(5 \times 0.45 = 2.25\,\text{L}\)
3
Total volume: \(5 + 2.25 = 7.25\,\text{L}\)
Worked Example: Cardiac Output
A pregnant woman has a resting heart rate of 88 bpm and a stroke volume of 80 mL. Calculate her cardiac output.
1
Formula: \(\text{CO} = \text{HR} \times \text{SV}\)
2
Substitute: \(\text{CO} = 88 \times 80 = 7{,}040\,\text{mL/min}\)
3
Convert: \(7{,}040\,\text{mL/min} = 7.04\,\text{L/min}\)
Common Mistakes
✕ Mixing Up hCG Source
hCG is secreted by the trophoblast (later placenta), not the ovary or the pituitary. The corpus luteum responds to hCG.
✕ Blood Pressure Should Rise
Despite increased blood volume, systemic vascular resistance decreases in pregnancy, so blood pressure actually drops in the 2nd trimester.
✕ Maternal and Fetal Blood Mix
They do not. Exchange occurs across the placental barrier. Rh sensitization only happens when fetal blood leaks during delivery or trauma.
Systems Summary
| System | Change | Mechanism |
|---|---|---|
| Cardiovascular | CO ↑ 30-50% | ↑ HR + ↑ SV + ↑ blood volume |
| Respiratory | Tidal volume ↑ 40% | Progesterone stimulates brainstem |
| Renal | GFR ↑ 50% | ↑ renal plasma flow |
| Endocrine | hCG, hPL, relaxin | Placental hormone production |
| Hematologic | Plasma ↑ > RBC ↑ | Physiological anemia |
Question 1 of 5
What hormone produced by the trophoblast maintains the corpus luteum during early pregnancy?
hCG is produced by the trophoblast after implantation and signals the corpus luteum to continue secreting progesterone and estrogen until the placenta takes over at ~weeks 8-12.
Question 2 of 5
During pregnancy, cardiac output increases primarily due to increases in both:
Cardiac output = heart rate \(\times\) stroke volume. Both increase during pregnancy. Peripheral resistance actually decreases, and blood viscosity drops due to hemodilution.
Question 3 of 5
The "physiological anemia of pregnancy" occurs because:
Both plasma and RBCs increase, but plasma increases more (~50% vs. ~30%), diluting the hematocrit. This is a normal adaptation, not a pathological anemia.
Question 4 of 5
Labor contractions are amplified by a positive feedback mechanism involving oxytocin. What event terminates this feedback loop?
Positive feedback loops require the stimulus to be removed for termination. Once the baby is delivered, the cervix is no longer being stretched, and the oxytocin feedback loop stops.
Question 5 of 5
During which developmental period is the embryo most vulnerable to teratogens causing structural birth defects?
Weeks 3-8 is when organogenesis occurs: all major organ systems are forming. Teratogen exposure during this critical window has the highest risk of causing major structural abnormalities.