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1
Question:

A 35-year-old woman, gravida 1 para 0, comes to the office for an initial prenatal visit.  The patient has had no vaginal bleeding or abdominal pain and has not yet felt fetal movement.  Menarche occurred at age 13.  She sometimes has heavy menstrual bleeding with passage of clots, and other times she skips her menses.  The patient's estimated gestational age based on her last menstrual period is 16 weeks.  She has type 1 diabetes mellitus that is controlled with insulin.  She smokes a pack of cigarettes daily.  As part of her prenatal laboratory screening, a second-trimester maternal serum quadruple screen is performed and reveals an elevated alpha-fetoprotein level.  Which of the following is the most likely etiology of this patient's abnormal screening result?

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Explanation:

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Maternal serum α-fetoprotein screening

↑ MSAFP

↓ MSAFP

  • Open neural tube defects (eg, anencephaly, open spina bifida)
  • Ventral wall defects (eg, omphalocele, gastroschisis)
  • Multiple gestation
  • Aneuploidies (eg, trisomy 18 & 21)

MSAFP = maternal serum α-fetoprotein.

The maternal serum quadruple screen is used to identify pregnancies at risk for congenital defects or fetal aneuploidy (eg, Down syndrome).  It measures the concentration of alpha-fetoprotein (AFP), estriol, β-hCG, and inhibin A in the maternal blood.

AFP is a glycoprotein produced in the fetal liver and gastrointestinal tract.  Levels are dependent on gestational age and maternal conditions such as diabetes mellitus (typically associated with decreased MSAFP and estriol levels).  Therefore, accurate pregnancy dating and complete medical history are required to correctly interpret an AFP level.

The most common cause of an abnormal AFP level is inaccurate pregnancy dating (ie, dating error).  In patients with irregular menses, dating by a last menstrual period can underestimate the true gestational age.  Therefore, these patients require a fetal ultrasound, which can accurately determine gestational age and evaluate for other common causes of elevated MSAFP levels, which include multiple gestation (eg, twin pregnancy), open neural tube defects, and abdominal wall defects.

(Choices B and C)  Down syndrome (trisomy 21) and Edwards syndrome (trisomy 18) are associated with low AFP levels.

(Choice D)  Fetal growth restriction can occur due to tobacco use, which causes placental ischemia and resultant placental insufficiency.  However, maternal serum quadruple screen shows decreased estriol levels, not increased AFP levels, with fetal growth restriction.

(Choice E)  Although the risk for fetal heart defects is increased with maternal diabetes mellitus, heart defects are not associated with abnormal AFP levels because AFP is not produced in fetal cardiac tissue.

(Choice F)  Hydatidiform moles and other gestational trophoblastic diseases typically present with markedly elevated β-hCG levels, not elevated AFP levels.  Patients may have vaginal bleeding and a significantly enlarged uterus.

Educational objective:
The maternal serum quadruple screen assesses for risk of congenital defects and fetal aneuploidy and includes measurement of alpha-fetoprotein levels, which are dependent on gestational age.  An abnormal alpha-fetoprotein level is most commonly due to a dating error (eg, inaccurate gestational age).