8.1 Obstetrics: Normal Pregnancy & Prenatal Care

Obstetrics: Normal Pregnancy & Prenatal Care

Why This Matters for PANCE

Maternal prenatal care, physiologic changes of pregnancy, and gestational screening protocols represent critical, high-yield topics on the PANCE. Questions frequently assess your ability to recognize normal physiologic adaptations versus pathological states, calculate estimated delivery dates, and apply the screening calendar (such as Rh antibody screening and Group B Streptococcus prophylaxis) to clinical scenarios.

Maternal Physiological Changes of Pregnancy

Pregnancy triggers profound anatomical and physiological adaptations across almost every organ system to support fetal development and prepare for delivery:

• Cardiovascular System: Maternal blood volume increases by 50% to optimize placental perfusion. However, plasma volume increases out of proportion to red blood cell mass (which increases by 20-30%), leading to a physiological hemodilution known as the dilutional anemia of pregnancy. Cardiac output increases by 30-50% due to an increase in both stroke volume and heart rate. Systemic vascular resistance (SVR) decreases significantly due to progesterone-induced vascular smooth muscle relaxation and the creation of the low-resistance uteroplacental circuit. Consequently, blood pressure drops in the second trimester (systolic by 5-10 mmHg, diastolic by 10-15 mmHg) before returning to pre-pregnancy baselines by term. A systolic murmur (usually grade I or II) is common and physiological due to hyperdynamic flow.
• Pulmonary System: Progesterone acts as a direct respiratory stimulant, increasing tidal volume by 30-40% while respiratory rate remains unchanged. This leads to an increase in minute ventilation, causing a mild, chronic, compensated respiratory alkalosis (arterial pH of 7.40-7.45 and pCO2 of 28-32 mmHg). This respiratory shift creates a favorable concentration gradient for carbon dioxide transport from the fetus to the mother. Anatomic changes include diaphragmatic elevation of approximately 4 cm, which decreases functional residual capacity (FRC) and residual volume.
• Gastrointestinal System: High circulating levels of progesterone induce smooth muscle relaxation throughout the GI tract. Lower esophageal sphincter (LES) tone is decreased, leading to gastroesophageal reflux disease (GERD). Gastric emptying and intestinal transit times are delayed, which increases water absorption and causes constipation. Gallbladder emptying is slowed, promoting biliary stasis and an increased risk of cholelithiasis. High levels of estrogen and beta-hCG in the first trimester contribute to nausea and vomiting of pregnancy ("morning sickness").
• Renal System: Renal blood flow and glomerular filtration rate (GFR) increase by 50%. This hyperfiltration causes serum creatinine and blood urea nitrogen (BUN) levels to decline; a normal serum creatinine in pregnancy is 0.4-0.6 mg/dL, and a value of 0.9 mg/dL is abnormally elevated. Progesterone-mediated relaxation of the ureteric smooth muscle, combined with mechanical compression by the gravid uterus, leads to physiologic hydroureter and hydronephrosis (more pronounced on the right side due to uterine dextrorotation). This urinary stasis increases the risk of progression from asymptomatic bacteriuria to acute pyelonephritis.
• Hematologic System: Pregnancy is a hypercoagulable state, an evolutionary adaptation to minimize blood loss during delivery. There is an increase in coagulation factors (fibrinogen, factors VII, VIII, IX, X, and XII) and a decrease in natural anticoagulants like protein S. Fibrinolytic activity is also suppressed. These alterations significantly elevate the risk of deep vein thrombosis (DVT) and pulmonary embolism (PE) throughout pregnancy and the postpartum period.

Gestational Age Calculation & Fetal Growth Milestones

• Naegele's Rule: The standard clinical method for calculating the Estimated Date of Delivery (EDD) based on a 28-day cycle: $\text{EDD} = \text{First day of Last Menstrual Period (LMP)} + 7\text{ days} - 3\text{ months} + 1\text{ year}$ Example: If a patient's LMP was October 10, 2025, adding 7 days (October 17) and subtracting 3 months gives an EDD of July 17, 2026.
• Ultrasound Dating: First-trimester ultrasound measuring the Crown-Rump Length (CRL) between 7 and 14 weeks is the gold standard for gestational dating, accurate within 3-5 days. If a discrepancy exists between LMP dating and CRL dating, the ultrasound dating should be used. Accuracy of ultrasound dating decreases in the second (14-22 weeks, accurate to ±7-10 days) and third trimesters (after 22 weeks, accurate to ±14-21 days) when parameters like biparietal diameter, head circumference, abdominal circumference, and femur length are measured.
• Fundal Height Milestones:
  • 12 weeks: The uterine fundus emerges from the pelvis and is palpable at the pubic symphysis.
  • 16 weeks: The fundus is palpable midway between the pubic symphysis and the umbilicus.
  • 20 weeks: The fundus reaches the level of the umbilicus. From 20 to 36 weeks, the fundal height in centimeters corresponds approximately 1:1 with the gestational age in weeks (e.g., at 28 weeks, the fundus should measure 28 cm from the pubic symphysis).
  • 36 weeks: The fundus reaches the xiphoid process.
  • 38-40 weeks: The fundal height may decrease slightly as the fetal head engages into the pelvis (a process called "lightening").

Prenatal Screening Calendar & Prophylaxis

Gestational Diabetes screening (24-28 weeks)

Screening uses a two-step approach. The initial test is a non-fasting 1-hour, 50-gram glucose challenge test. If the 1-hour venous glucose is >= 130-140 mg/dL (depending on institutional threshold), the patient must return for a fasting 3-hour, 100-gram oral glucose tolerance test (OGTT). A diagnosis of GDM is confirmed if two or more of the following values are met or exceeded (Carpenter-Coustan criteria):

• Fasting: >= 95 mg/dL
• 1-hour: >= 180 mg/dL
• 2-hour: >= 155 mg/dL
• 3-hour: >= 140 mg/dL

Rh Incompatibility and RhoGAM Prophylaxis (28 weeks)

If an Rh-negative mother carries an Rh-positive fetus, exposure to fetal red blood cells (usually during delivery or trauma) triggers maternal IgM then IgG antibody production against the Rh (D) antigen. In subsequent pregnancies, maternal IgG antibodies cross the placenta, causing hemolysis of fetal red cells (erythroblastosis fetalis/hydrops fetalis). To prevent this, RhoGAM (Rh immune globulin) 300 mcg IM is administered at 28 weeks gestation to Rh-negative, unsensitized mothers (with a negative indirect Coombs test). It is also given within 72 hours of delivery if the neonate is Rh-positive, and at any time fetomaternal hemorrhage is suspected (e.g., after miscarriage, ectopic pregnancy, amniocentesis, external cephalic version, or abdominal trauma).

Group B Streptococcus (GBS) Screening & Management (35-37 weeks)

Group B Streptococcus (Streptococcus agalactiae) is a transient colonizer of the maternal rectovaginal tract that can be transmitted during delivery, causing neonatal sepsis, pneumonia, or meningitis. All pregnant women undergo GBS screening via a vaginal-rectal swab at 35-37 weeks gestation.

• Intrapartum Prophylaxis Indication: If the screen is positive, the patient must receive intravenous Penicillin G during labor (first-line; ampicillin is a common alternative. Cefazolin is used for mild penicillin allergy; clindamycin or vancomycin for severe allergy depending on susceptibility). Prophylaxis is also indicated if a patient has GBS bacteriuria at any concentration during the current pregnancy (even if treated) or has a history of a previous infant affected by GBS disease.
• Unknown Status: If GBS status is unknown at delivery, administer prophylaxis if the patient has risk factors: preterm labor (<37 weeks), intrapartum fever (>= 100.4°F), or rupture of membranes >= 18 hours.

Classic PANCE Traps & Clinical Pearls

• Asymptomatic Bacteriuria: In non-pregnant patients, asymptomatic bacteriuria is not treated. In pregnant patients, it must always be treated to prevent acute pyelonephritis and associated preterm labor. First-line agents include cephalexin, nitrofurantoin (avoid in first trimester and at term), or amoxicillin-clavulanate. Avoid fluoroquinolones (cartilage toxicity) and tetracyclines (bone/tooth discoloration).
• RhoGAM in Sensitized Patients: If the maternal antibody screen (indirect Coombs test) is positive (e.g., titer >= 1:4), the mother is already sensitized, meaning she has active anti-D antibodies. In this scenario, administering RhoGAM is ineffective. Management must shift to serial ultrasounds of the fetus (middle cerebral artery Doppler velocities) to screen for fetal anemia.
• GBS Bacteriuria Bypass: If a patient has GBS bacteriuria in the current pregnancy, she does not undergo vaginal-rectal swab screening at 35-37 weeks. She is automatically designated to receive intrapartum IV penicillin G during labor.