A 36-year-old G3P2002 presents to her obstetrician’s office for her first prenatal visit at ten weeks and two days gestation. She notes that she has felt nauseous the last several mornings and has been especially tired for a few weeks. Otherwise, she feels well. The patient has had two uncomplicated spontaneous vaginal deliveries at full term with her last child born six years ago. She is concerned about the risk of Down syndrome in this fetus, as her sister gave birth to an affected child at age 43. The patient has a history of generalized anxiety disorder, atopic dermatitis, and she is currently on escitalopram. At this visit, this patient’s temperature is 98.6°F (37.0°C), pulse is 70/min, blood pressure is 121/67 mmHg, and respirations are 13/min. The patient appears anxious, but overall comfortable, and cardiopulmonary and abdominal exams are unremarkable. Pelvic exam reveals normal female external genitalia, a closed and slightly soft cervix, a ten-week-sized uterus, and no adnexal masses. Which of the following is the best next step for definitively determining whether this patient’s fetus has Down syndrome?

Genetic testing of patient’s sister

A 35-year-old woman gravida 2, para 1, comes to the physician for her first prenatal visit. Pregnancy and delivery of her first child were uncomplicated. She is not sure about the date of her last menstrual period. Pelvic examination shows a uterus consistent in size with a 10-week gestation. An ultrasound examination confirms the gestational age and shows one fetus with no indication of multiple gestations. During counseling on pregnancy risks and possible screening and diagnostic tests, the patient states she would like to undergo screening for Down syndrome. She would prefer immediate and secure screening with a low risk to herself and the fetus. Which of the following is the most appropriate next step in management at this time?

Nuchal translucency, pregnancy-associated plasma protein-A, human chorionic gonadotropin

Maternal serum α-fetoprotein, human chorionic gonadotropin, unconjugated estriol, and inhibin A

A 38-year-old woman, gravida 2, para 1, at 24 weeks' gestation comes to the physician for a routine prenatal evaluation. She has no history of major medical illness and takes no medications. Fetal ultrasonography shows a cardiac defect resulting from abnormal development of the endocardial cushions. This defect is most likely to result in which of the following?

Transposition of the great vessels

A 28-year-old primigravid woman comes to the emergency department because of a 12-hour history of lower abdominal pain and vaginal bleeding. She also had nausea and fatigue for the past 3 weeks. Her last menstrual period was 8 weeks ago. Prior to that, her menses occurred regularly at 30-day intervals and lasted for 4 days. There is no history of medical illness, and she takes no medications. Her temperature is 37°C (98.6°F), pulse is 95/min, and blood pressure is 100/70 mm Hg. Pelvic examination is painful and shows a uterus consistent in size with a 13-week gestation. A urine pregnancy test is positive. β-HCG level is 106,000 mIU/mL (N < 5 mIU/mL). Transvaginal ultrasonography shows unclear, amorphous fetal parts and a large placenta with multiple cystic spaces. Which of the following is the most likely cause of this patient's condition?

Trophoblastic proliferation with chorionic villi distention

Placenta implantation into myometrium

Embryonic death with cervical dilation

Malpositioned placenta overlying the cervix

Malignant transformation of trophoblastic tissue

A female presents with postcoital bleeding. Which of the following is the most appropriate investigation?

Pap smear, HCV DNA, electrophoresis

Liquid-based cytology, cervical biopsy

Second trimester serum screening for USMLE Step 3: High-Yield OB/GYN Lessons

Quad Screen - The Core Four

Performed at 15-22 weeks. Measures four maternal serum markers: AFP, hCG, Estriol, and Inhibin A. It screens for Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and neural tube defects (NTDs).

Quadruple Screen Report for Trisomy 21, 18, and NTD

Condition	AFP	hCG	Estriol	Inhibin A
Trisomy 21	↓	↑↑	↓	↑
Trisomy 18	↓	↓	↓	N/A
NTDs / Abdominal Wall Defects	↑	N/A	N/A	N/A

⭐ Inhibin A is the component that increases sensitivity and differentiates the quad screen from the older triple screen.

Result Patterns - Decoding the Clues

The Quad Screen measures maternal serum AFP, hCG, Estriol, and Inhibin A. Patterns point to specific fetal abnormalities.

Condition	AFP	hCG	Estriol	Inhibin A
Trisomy 21 (Down Syn.)	↓	↑	↓	↑
Trisomy 18 (Edwards Syn.)	↓	↓	↓	Normal
NTD / Abd. Wall Defect	↑	N/A	N/A	N/A

⭐ In Trisomy 21, hCG and Inhibin A are characteristically elevated. Inhibin A is the key marker that differentiates the Quad Screen from the older Triple Screen, increasing detection rates.

Clinical Pathway - The Next Steps

Initial Step: Detailed Ultrasound
- Confirm gestational age; incorrect dating is a common cause of false positives.
- Rule out multiple gestations (which alters marker levels).
- Perform a detailed fetal anatomy survey for structural anomalies (e.g., NTDs, cardiac defects).
Genetic Counseling
- Discuss screening results, individual risk, and the risks/benefits of diagnostic testing.
Definitive Diagnosis
- If risk remains high after ultrasound, offer invasive testing for a definitive karyotype.
- Amniocentesis: The standard diagnostic test in the 2nd trimester, performed at 15-20 weeks.

⭐ For an unexplained elevated MSAFP, a targeted ultrasound is key. If the anatomy scan is normal, most pregnancies have a normal outcome, though there's an increased risk for placental insufficiency and IUGR.

High-Yield Points - ⚡ Biggest Takeaways

The quad screen, performed at 15-22 weeks, screens for neural tube defects (NTDs) and aneuploidy.

It measures four maternal serum markers: AFP, hCG, estriol, and inhibin A.

In Trisomy 21, expect ↓ AFP and estriol, but ↑ hCG and inhibin A.

In Trisomy 18, expect ↓ AFP, ↓ hCG, and ↓ estriol.

An isolated ↑ AFP suggests an NTD like spina bifida or anencephaly.

This is a screening test, not diagnostic; abnormal results need follow-up with ultrasound or amniocentesis.

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