A 36-year-old G3P2002 woman presents to her obstetrician’s office for her 1st prenatal visit at 10 weeks and 2 days gestation. She has felt nauseous the last several mornings and has been tired for a few weeks. The patient has had 2 uncomplicated spontaneous vaginal deliveries at full term with her last child born 6 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. She has a history of generalized anxiety disorder, atopic dermatitis, and she is currently on escitalopram. Her 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 external genitalia, a closed and soft cervix, a 10-week-sized uterus, and no adnexal masses. Which of the following is the most appropriate next step for definitively determining whether this patient’s fetus has Down syndrome?
Anatomy ultrasound
Genetic testing of patient’s sister
Nuchal translucency test
Chorionic villus sampling
Chorionic villus sampling
This patient is over 35 years of age and concerned about the risk of Down syndrome (trisomy 21) in her fetus at just over 10 weeks gestation. At this gestational age, the most definitive method for determining whether such an aneuploidy is present is chorionic villus sampling (CVS).

CVS, or sampling of the placental tissue, can be performed as early as 10 weeks gestation. This is in contrast to amniocentesis, which should be avoided prior to 14 weeks. There are other screening tests for a patient concerned about aneuploidy; however, only CVS and amniocentesis offer definitive diagnoses. Complications of this procedure include pregnancy loss (at a rate of 1-2%), transverse limb abnormality, rupture of membranes, and chorioamnionitis. Patients should also have counseling if genetic abnormalities are detected using this procedure.

Farina reviews the evidence regarding the complications associated with CVS. He discusses how this procedure is becoming more common as the definitive method of providing an early diagnosis. He recommends being aware of complications such as possible pregnancy loss.

Incorrect Answers:
Answer A: Amniocentesis is a useful tool for the definitive diagnosis of aneuploidy and other genetic conditions. Due to the higher rate of fetal loss at younger gestational ages, it is reserved for those above 14 weeks.

Answer B: The anatomy ultrasound can be helpful in identifying morphologic features that are concerning for Down syndrome, such as a flat nasal bridge or an echogenic cardiac focus. This screening tool is most accurate after 18 weeks gestation.

Answer D: Genetic testing of the patient’s sister may be able to help identify if there is a parental translocation or aneuploidy that led to the child’s Down syndrome. This is much less likely than meiotic nondisjunction due to advanced maternal age, and this would not help diagnose Down syndrome in the patient’s fetus.

Answer E: The nuchal translucency test examines the thickness of the nuchal fold. When the thickness exceeds normal measurements, there is an increased risk of aneuploidy such as Down syndrome, but this is a screening result that would not provide a definitive diagnosis.

Bullet Summary:
Chorionic villus sampling is a definitive method of identifying aneuploidy in fetuses starting at 10 weeks of gestational age.