A 39-year-old woman, gravida 2 para 1, at 38 weeks gestation, comes to the office for a routine prenatal visit. The patient feels well and reports normal fetal activity. Last week, she had nausea, vomiting, and diarrhea for 24 hours, but she has been able to eat and drink normally for the past several days. For the past 2 days, the patient has had increased clear vaginal discharge with irregular, painful contractions but no vaginal bleeding. She has no chronic medical conditions, and her only medication is a prenatal vitamin. Temperature is 99.6 F (37.6 C), blood pressure is 132/86 mm Hg, and pulse is 96/min. The uterus is nontender and the fundal height is 34 cm. Fetal heart tones are 135/min. Fetal ultrasound shows a cephalic fetus, measuring at the 50th percentile for gestational age, and an amniotic fluid index of 2 cm (normal: 5 cm). Which of the following is the most likely cause of this patient's amniotic fluid level?

Question

A 39-year-old woman, gravida 2 para 1, at 38 weeks gestation, comes to the office for a routine prenatal visit.  The patient feels well and reports normal fetal activity.  Last week, she had nausea, vomiting, and diarrhea for 24 hours, but she has been able to eat and drink normally for the past several days.  For the past 2 days, the patient has had increased clear vaginal discharge with irregular, painful contractions but no vaginal bleeding.  She has no chronic medical conditions, and her only medication is a prenatal vitamin.  Temperature is 99.6 F (37.6 C), blood pressure is 132/86 mm Hg, and pulse is 96/min.  The uterus is nontender and the fundal height is 34 cm.  Fetal heart tones are 135/min.  Fetal ultrasound shows a cephalic fetus, measuring at the 50th percentile for gestational age, and an amniotic fluid index of 2 cm (normal: >5 cm).  Which of the following is the most likely cause of this patient's amniotic fluid level?

Quizplus · Accepted Answer

Explanation: This patient has oligohydramnios, an amniotic fluid index of ≤5 cm. The etiology of oligohydramnios can vary with gestational age: Early-gestation oligohydramnios is concerning for fetal etiologies (eg, aneuploidy, renal agenesis, posterior urethral valves) because amniotic fluid volume is dependent on normal fetal urine production. Second- and third-trimester causes of oligohydramnios are typically due to uteroplacental insufficiency (with concomitant fetal growth restriction) or maternal causes, such as dehydration or rupture of membranes (with normal fetal growth). At term gestation, the most likely cause of new-onset oligohydramnios is spontaneous rupture of membranes. Although membrane rupture typically presents as a sudden gush of fluid, patients can have a subclinical presentation with slow leakage occurring over days, as in this patient with increased clear vaginal discharge. Because rupture of membranes is a common cause of oligohydramnios, even asymptomatic patients with oligohydramnios require speculum examination to evaluate for membrane rupture. (Choice A) Although fetal congenital infections (eg, cytomegalovirus) can cause oligohydramnios, they typically also cause fetal growth restriction and other abnormal ultrasound findings, which are not seen in this patient. (Choice B) Severe maternal dehydration may temporarily decrease maternal intravascular volume, uteroplacental perfusion, and fetal amniotic fluid production. However, this patient's self-limited, acute gastroenteritis resolved several days ago, and her vital signs are normal, making this diagnosis less likely. (Choice C) The amniotic fluid index (AFI) varies with gestational age and peaks at 30-32 weeks gestation before gradually declining; however, a third-trimester AFI of ≤5 cm is always abnormal. (Choice E) Maternal vascular disease (eg, chronic hypertension, systemic lupus erythematosus) increases the risk for uteroplacental insufficiency, which causes oligohydramnios due to decreased perfusion of the fetal kidneys. However, these patients typically have concomitant fetal growth restriction, which is not seen in this patient. Educational objective: The most common cause of oligohydramnios (amniotic fluid index ≤5 cm) at term gestation is spontaneous rupture of membranes. Patients typically have normal fetal growth and leakage of clear vaginal fluid. References: Induction of labor compared with expectant management for prelabor rupture of the membranes at term. TERMPROM study group. (http://www.ncbi.nlm.nih.gov/pubmed/8598837) Premature rupture of membranes at term: early induction versus expectant management. (http://www.ncbi.nlm.nih.gov/pubmed/23543046)

A 39-Year-Old Woman, Gravida 2 Para 1, at 38 Weeks