A 36-year old pregnant woman (gravida 4, para 1) presents at week 11 of pregnancy. Currently, she has no complaints. She had an uncomplicated 1st pregnancy that ended in an uncomplicated vaginal delivery at the age of 28 years. Her male child was born healthy, with normal physical and psychological development over the years. Two of her previous pregnancies were spontaneously terminated in the 1st trimester. Her elder sister has a child born with Down syndrome. The patient denies smoking and alcohol consumption. Her blood analysis reveals the following findings: Measured values Beta human chorionic gonadotropin (beta-hCG) High Pregnancy-associated plasma protein-A (PAPP-A) Low Which of the following is the most appropriate next step in the management of this patient?

Recommend chorionic villus sampling with subsequent cell culturing and karyotyping

Perform an ultrasound examination with nuchal translucency and crown-rump length measurement

Offer a blood test for rubella virus, cytomegalovirus, and toxoplasma IgG

Recommend amniocentesis with subsequent cell culturing and karyotyping

Schedule a quadruple test at the 15th week of pregnancy

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 26-year-old pregnant woman (gravida 2, para 1) presents on her 25th week of pregnancy. Currently, she has no complaints. Her previous pregnancy was unremarkable. No abnormalities were detected on the previous ultrasound (US) examination at week 13 of pregnancy. She had normal results on the triple test. She is human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C virus (HCV)-negative. Her blood type is III(B) Rh+, and her partner has blood type I(0) Rh-. She and her husband are both of Sardinian descent, do not consume alcohol, and do not smoke. Her cousin had a child who died soon after the birth, but she doesn't know the reason. She does not report a history of any genetic conditions in her family, although notes that her grandfather “was always yellowish-pale, fatigued easily, and had problems with his gallbladder”. Below are her and her partner’s complete blood count and electrophoresis results. Complete blood count Patient Her husband Erythrocytes 3.3 million/mm3 4.2 million/mm3 Hb 11.9 g/dL 13.3 g/dL MCV 71 fL 77 fL Reticulocyte count 0.005 0.008 Leukocyte count 7,500/mm3 6,300/mm3 Platelet count 190,000/mm3 256,000/mm3 Electrophoresis HbA1 95% 98% HbA2 3% 2% HbS 0% 0% HbH 2% 0% The patient undergoes ultrasound examination which reveals ascites, liver enlargement, and pleural effusion in the fetus. Further evaluation with Doppler ultrasound shows elevated peak systolic velocity of the fetal middle cerebral artery. Which of the following procedures can be performed for both diagnostic and therapeutic purposes in this case?

Percutaneous fetal thoracentesis

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 34-year-old gravida 2 para 1 woman at 16 weeks gestation presents for prenatal care. Her prenatal course has been uncomplicated. She takes no medications besides her prenatal vitamin which she takes every day, and she has been compliant with routine prenatal care. She has a 7-year-old daughter who is healthy. The results of her recent quadruple screen are listed below: AFP: Low hCG: Low Estriol: Low Inhibin-A: Normal Which of the following is the most appropriate next step to confirm the diagnosis?

Ultrasound for nuchal translucency

A 31-year-old G1P0 woman at 26 weeks gestation presents to the clinic for evaluation of an abnormal glucose tolerance test. She denies any symptoms, but states that she was given 50 g of oral glucose 1 week earlier and demonstrated a subsequent venous plasma glucose level of 156 mg/dL 1 hour later. The vital signs are: blood pressure, 112/78 mm Hg; pulse, 81/min; and respiratory rate, 16/min. Physical examination is within normal limits. Which of the following is the most appropriate next step in management?

Administer an oral, 3-hour 100 g glucose dose

Advise the patient to follow an American Diabetic Association diet plan

Repeat the 50 g oral glucose challenge

Order a fetal ultrasound examination

A 26-year-old woman presents to the women’s health clinic with a 9-week delay in menses. The patient has a history of grand mal seizures, and was recently diagnosed with acute sinusitis. She is prescribed lamotrigine and amoxicillin. The patient smokes one-half pack of cigarettes every day for 10 years, and drinks socially a few weekends every month. Her mother died of breast cancer when she was 61 years old. The vital signs are stable during the current office visit. Physical examination is grossly normal. The physician orders a urine beta-hCG that comes back positive. Abdominal ultrasound shows an embryo consistent in dates with the first day of last menstrual period. Given the history of the patient, which of the following would most likely decrease congenital malformations in the newborn?

Switching to another antiepileptic medication

Diagnostic testing (amniocentesis, CVS) for USMLE Step 3: High-Yield OB/GYN Lessons

Indications for Invasion - Why We Test

Advanced Maternal Age (AMA): Age ≥ 35 at delivery.
Abnormal non-invasive screening test result (e.g., cfDNA, quad screen).
Previous pregnancy with a documented chromosomal abnormality.
Abnormal ultrasound findings:
- Structural defects (e.g., cardiac, neural tube).
- Soft markers (e.g., ↑ nuchal translucency, absent nasal bone).
Parental carrier of a chromosomal abnormality (e.g., balanced translocation).

📌 Mnemonic: AMAAP

⭐ The most common indication for invasive testing is now an abnormal cell-free DNA (cfDNA) result, surpassing AMA in many centers.

Normal vs. Increased Nuchal Translucency Ultrasound

Chorionic Villus Sampling - The Early Peek

Procedure: Transcervical or transabdominal aspiration of chorionic villi (placental tissue) for genetic/chromosomal analysis.
Timing: Performed early at 10-13 weeks gestation.
Advantages: Provides the earliest definitive genetic diagnosis, allowing more time for parental decision-making.
Risks & Limitations:
- Slightly higher risk of fetal loss (~1%) vs. amniocentesis.
- ⚠️ Risk of transverse limb defects if performed before 10 weeks.
- Cannot detect Neural Tube Defects (NTDs).
- Potential for confined placental mosaicism, which may require follow-up amniocentesis.

⭐ High-Yield: Unlike amniocentesis, CVS does not analyze amniotic fluid. Therefore, it cannot detect open neural tube defects, which require measuring alpha-fetoprotein (AFP).

Transvaginal Chorionic Villus Sampling (CVS)

Amniocentesis - The Fluid Truth

Ultrasound-guided amniocentesis procedure

Procedure: Transabdominal needle aspiration of amniotic fluid using ultrasound guidance, collecting fetal cells (amniocytes) for analysis.
Timing: Performed between 15-20 weeks gestation. Can be done later for fetal lung maturity assessment.
Indications:
- Advanced maternal age (≥35).
- Abnormal aneuploidy screening results (e.g., first-trimester screen, quad screen).
- Parental carrier of a chromosomal translocation.
- History of a child with a structural birth defect or chromosomal abnormality.
Risks:
- Procedure-related fetal loss rate is low, approx. 1/900.
- ⚠️ Potential for membrane rupture, infection, or needle injury to the fetus.

⭐ Amniotic fluid acetylcholinesterase (AChE) is a highly specific marker for open neural tube defects.

Procedural Face-Off - CVS vs. Amnio

Amniocentesis vs. CVS procedure and karyotyping

Feature	Chorionic Villus Sampling (CVS)	Amniocentesis
Timing	10-13 weeks	15-20 weeks
Sample	Placental villi (chorion frondosum)	Amniotic fluid with fetal cells
Route	Transcervical or transabdominal	Transabdominal
Risk	Fetal loss (~1%); ↑ risk of limb reduction defects if <10 wks	Fetal loss (~0.1-0.3%)
Pros	Earlier diagnosis	Safer; can measure AFP
Cons	Cannot detect NTDs; risk of maternal cell contamination	Later diagnosis

High‑Yield Points - ⚡ Biggest Takeaways

Amniocentesis is performed at 15-20 weeks; Chorionic Villus Sampling (CVS) is done earlier, at 10-13 weeks.

Only amniocentesis can detect neural tube defects by measuring alpha-fetoprotein (AFP).

CVS has a slightly higher risk of fetal loss and can cause limb reduction defects if performed before 10 weeks.

Amniocentesis samples fetal cells from amniotic fluid; CVS samples placental tissue.

Confined placental mosaicism is a potential diagnostic pitfall specific to CVS.

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