Obstetrics - USMLE

• Gravidity


  Number of times a woman has been pregnant
• Parity


  Number of pregnancies that led to a birth beyond 20 weeks' gestational age or an infant weighing >500 g
• TPAL


  Expresses the number of term deliveries (T), the number of preterm deliveries (P), the number of abortuses (A), and the number of living children (L)
• Embryonic age


  Number of weeks + days since fertilization; usually unknown
• Gestational age (GA)


  The number of weeks and days measured from the first day of the last menstrual period (LMP)
• Gestational age determination


  • Fundal height
  • Fetal heart tones (Doppler)
  • Quickening or appreciation of fetal movement
  • Ultrasonography
• 1. human chorionic gonadotropin (B-hCG)


  The standard for diagnosing pregnancy, can be detected in serum or urine
• Serum B-hCG is more sensitive and preferred if menstrual period is <1 week late
• 1. hCG is produced by the placenta, peaks at 100,000 mIU/mL by 10 weeks and decreases throughout the second and third trimesters
• 1. hCG levels double approximately every 48 hours during early pregnancy; failure of hCG levels to double every 48 hours is concerning for miscarriage or ectopic pregnancy
• Ultrasonography


  Used to confirm an intrauterine pregnancy, gestational sac visible on transvaginal ultrasonography by 5 weeks and a B-hCG in the range of 1500 to 3500 mIU/mL
• Transabdominal ultrasound (US) is typically reserved for second-/third-trimester measurements
• Physiologic changes in pregnancy

  • Cardiovascular
  • Circulatory
  • Pulmonary
  • Renal
  • Gastrointestinal
  • Musculoskeletal
  • Skin
  • Endocrine
• The goal of prenatal care is to prevent, diagnose, and treat conditions that can lead to adverse fetal or maternal outcomes in pregnancy
• Recommendations for standard prenatal care

  • Weight gain
  • Nutrition
  • Exercise
  • Harmful substances
• Group B Streptococcus (GBS) testing and treatment


  • Screening for GBS: Rectovaginal swab at 36 to 38 weeks
  • Indications for intrapartum prophylaxis
  • Intrapartum prophylaxis: Intravenous (IV) penicillin
• Prenatal diagnostic testing schedule

  • Initial visit
  • 10-22 weeks
  • 18-20 weeks
  • 24-28 weeks
  • 28-30 weeks
  • 35-37 weeks
  • 34-40 weeks
• Quadruple screening


  Consists of maternal serum a-fetoprotein (MSAFP), inhibin A, estriol, and B-hCG
• MSAFP is produced by the fetus and enters the maternal circulation
• Challenge test


  1. Rh (D antigen) immune globulin for Rhe women (after antibody screen)

  2. 24-28 weeks

  3. 28-30 weeks

  4. 35-37 weeks

  5. GBS culture; repeat CBC

  6. 34-40 weeks

  7. In high-risk patients, cervical chlamydia and gonorrhea cultures, HIV, RPR
• Rh immune globulin to treat any unsensitized Rhe woman during any occasion of fetal-maternal blood mixing (eg, spontaneous abortion [SAB], placental abruption, abdominal trauma) even if <28 weeks' gestation
• Quadruple Screening for Fetal Aneuploidy

  • MSAFP
  • Trisomy 18
  • Trisomy 21
  • Estriol
  • Inhibin A
  • B-HCG
• MSAFP


  Produced by the fetus and enters the maternal circulation. Results are reported as multiples of the median (MoMs). Measurement results depend on accurate gestational dating. Multiple gestations and uterine leiomyomata (fibroids) may cause size/date discrepancy.
• MSAFP is rarely tested alone, as quad screening has ↑ sensitivity for detecting chromosomal abnormalities.
• Conditions associated with ↑ MSAFP (>2.5 MoMs)

  • Open neural tube defects (anencephaly, spina bifida)
  • Abdominal wall defects (gastroschisis, omphalocele)
  • Multiple gestation
  • Incorrect gestational dating
  • Fetal death
  • Placental abnormalities (eg, placental abruption)
• Conditions associated with ↓ MSAFP (<0.5 MoMs)

  • Trisomies 21 and 18
  • Incorrect gestational dating
• Nuchal Translucency


  Recommended at weeks 9 to 14. PAPP-A + nuchal translucency + free ẞ-hCG can detect ~91% of cases of Down syndrome and ~95% of cases of trisomy 18.
• Nuchal Translucency


  • Available earlier than chorionic villus sampling (CVS) and less invasive than CVS
• Prenatal Screening for Fetal Genetic Abnormalities

  • Cell-free fetal DNA
  • Chorionic Villus Sampling
  • Amniocentesis
• Cell-free fetal DNA


  Isolation of fetal DNA from blood sample obtained from pregnant patient
• Chorionic Villus Sampling


  Transcervical or transabdominal aspiration of placental tissue
• Amniocentesis


  Transabdominal aspiration of amniotic fluid, using an ultrasound-guided needle
• Indications for Amniocentesis


  • Concern for fetal genetic disease based on abnormal aneuploidy screening and/or ultrasound findings
  • Rh-sensitized pregnancy to obtain fetal blood type or to detect fetal hemolysis
  • Evaluation of fetal lung maturity
  • Assess for intraamniotic infection
• Common Teratogenic Agents and Their Associated Defects

  • Angiotensin-converting enzyme (ACE) inhibitors
  • Alcohol
  • Amphetamines
  • Androgens
  • Carbamazepine
  • Cocaine
  • Diethylstilbestrol (DES)
  • Lead
  • Lithium
  • Methotrexate
  • Organic mercury
  • Phenytoin
  • Radiation
  • Streptomycin and kanamycin
  • Tetracyclines
  • Thalidomide
  • Trimethadione and paramethadione
  • Valproic acid
  • Vitamin A and derivatives
  • Warfarin (wages war on the fetus)
• Congenital Infections


  • Toxoplasmosis
  • Rubella
  • Cytomegalovirus
  • HIV
  • Herpes viruses (herpes simplex virus, varicella zoster virus)
  • Syphilis
  • Zika virus
• Pregnant patients should not change a cat's litterbox to prevent exposure to toxoplasma.
• Diagnosis and Treatment of Common Congenital Infections

  • Toxoplasmosis
  • Rubella
  • CMV
  • HSV
  • HIV
  • Syphilis
  • Zika virus
• Types of Spontaneous Abortion

  • Complete
  • Threatened
  • Incomplete
  • Inevitable
  • Missed
• More than 80% of cases of spontaneous abortion occur in the first trimester.
• Associations with Spontaneous Abortion

  • Maternal factors: Inherited thrombophilia, Immunologic issues, Anatomic issues, Endocrinologic issues, Genetics, Other
  • Environmental factors: Tobacco, alcohol, excessive caffeine, toxins, drugs, radiation
  • Fetal factors: Anatomic malformation, Chromosomal abnormalities