A 46-year-old woman has a singleton pregnancy from IVF with a donated egg; the donor was 26 at egg collection. At 12 weeks her first-trimester combined screen gives nuchal translucency 1.7 mm (normal for the crown-rump length), free beta-hCG 1.05 MoM and PAPP-A 0.98 MoM. The risk software, which was given the recipient's age, reports a trisomy 21 risk of 1 in 95; the program calls a risk greater than 1 in 150 screen-positive. The midwife asks why the result is positive when every marker is normal. What should the laboratory do?

Recalculate the risk using the donor's age

Report it as screen-positive and offer CVS

Repeat the blood markers in two weeks

Switch to a second-trimester quad screen

Why are maternal serum screening results adjusted for the mother's weight?

Heavier women dilute the markers in a larger blood volume

Lighter women clear the markers faster through the kidney

Heavier women make more AFP in their own liver

Body fat interferes with the immunoassay signal

Prenatal screening results are reported as multiples of the median (MoM). A MoM is the patient's result divided by the:

Median for unaffected pregnancies at the same week

Upper reference limit in non-pregnant women

Mean of the laboratory's control results

Mean for all pregnant women at any stage

Pregnancy and prenatal screening

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Pregnancy and prenatal screening, part of Clinical Chemistry. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 46-year-old woman has a singleton pregnancy from IVF with a donated egg; the donor was 26 at egg collection. At 12 weeks her first-trimester combined screen gives nuchal translucency 1.7 mm (normal for the crown-rump length), free beta-hCG 1.05 MoM and PAPP-A 0.98 MoM. The risk software, which was given the recipient's age, reports a trisomy 21 risk of 1 in 95; the program calls a risk greater than 1 in 150 screen-positive. The midwife asks why the result is positive when every marker is normal. What should the laboratory do?
1. ARecalculate the risk using the donor's age
2. BReport it as screen-positive and offer CVS
3. CRepeat the blood markers in two weeks
4. DSwitch to a second-trimester quad screen
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Answer: A · Recalculate the risk using the donor's age
The prior risk of trisomy 21 depends on the age of the egg, not the age of the woman carrying the pregnancy. With all markers near 1.0 MoM and a normal nuchal translucency, the combined likelihood ratio is low, so the high reported risk comes almost entirely from the prior risk of a 46-year-old. Using the donor's age at egg collection gives a far lower prior risk, and the recalculated result will very likely fall below the cut-off. Repeating the markers or changing to another screen does not fix wrong demographic data, and a false-positive report could lead to an unnecessary invasive test. Request forms should record egg donation and the donor's age.
- B: The positive result comes from the wrong age, so reporting it could lead to an unnecessary invasive test.
- C: The markers are already normal; repeating them does not correct the age used.
- D: A second-trimester screen would also need the donor's age; changing tests does not fix the input error.
Question 2 · Medium
Why are maternal serum screening results adjusted for the mother's weight?
1. ALighter women clear the markers faster through the kidney
2. BHeavier women dilute the markers in a larger blood volume
3. CHeavier women make more AFP in their own liver
4. DBody fat interferes with the immunoassay signal
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Answer: B · Heavier women dilute the markers in a larger blood volume
The fetus and placenta release about the same amount of each marker whatever the mother's size, but a heavier woman has a larger plasma volume. Her marker concentrations are therefore lower, and without correction her risk would be misjudged. Laboratories apply a weight correction to the MoM values, usually from a regression equation. An accurate maternal weight must therefore be recorded on the request form.
- A: Marker levels relate to plasma volume, not to faster kidney clearance.
- C: AFP in pregnancy comes from the fetus, not from the mother's liver.
- D: The effect is physiological dilution, not analytical interference.
Question 3 · Hard
At 12 weeks, a 38-year-old woman with a singleton pregnancy has first-trimester combined screening: nuchal translucency 3.6 mm (above the 99th centile), free beta-hCG 2.4 MoM and PAPP-A 0.35 MoM. These results most increase the risk of:
1. ATrisomy 13
2. BOpen neural tube defect
3. CTrisomy 21
4. DTrisomy 18
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Answer: C · Trisomy 21
In trisomy 21, first-trimester free beta-hCG is typically about twice the median and PAPP-A about half the median, and the nuchal translucency is often increased. In trisomies 18 and 13, both free beta-hCG and PAPP-A are low. The combined test gives a risk, not a diagnosis, so a high-risk result is followed by cell-free DNA screening or by chorionic villus sampling or amniocentesis. First-trimester serum markers do not screen for open neural tube defects.
- A: Trisomy 13 also lowers both free beta-hCG and PAPP-A.
- B: Neural tube defects are screened by second-trimester AFP or ultrasound, not these markers.
- D: Trisomy 18 lowers both free beta-hCG and PAPP-A.
Question 4 · Medium
At 17 weeks, dated by ultrasound, a woman carrying one fetus has maternal serum AFP 3.2 MoM (screen-positive cut-off 2.5 MoM). Her hCG, unconjugated estriol and inhibin A are all near 1.0 MoM. Which condition does this result most suggest?
1. ATrisomy 21
2. BTrisomy 18
3. COpen neural tube defect
4. DSmith-Lemli-Opitz syndrome
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Answer: C · Open neural tube defect
With accurate ultrasound dating and a single fetus, an isolated raised AFP points to an open defect such as spina bifida or anencephaly, through which AFP leaks into the amniotic fluid and then the maternal blood. An abdominal wall defect such as gastroschisis can give the same result. The next step is a detailed ultrasound scan. Trisomies 21 and 18 lower AFP rather than raise it.
- A: Trisomy 21 gives low AFP and estriol with raised hCG and inhibin A.
- B: Trisomy 18 gives low AFP, estriol and hCG.
- D: Smith-Lemli-Opitz syndrome gives a very low estriol, not a raised AFP.
Question 5 · Medium
Prenatal screening results are reported as multiples of the median (MoM). A MoM is the patient's result divided by the:
1. AUpper reference limit in non-pregnant women
2. BMedian for unaffected pregnancies at the same week
3. CMean of the laboratory's control results
4. DMean for all pregnant women at any stage
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Answer: B · Median for unaffected pregnancies at the same week
Marker levels change week by week, so each result is divided by the median for unaffected pregnancies at the same gestational age. A value of 1.0 MoM is exactly typical, which lets results be compared across weeks and between laboratories. The MoM is then adjusted for factors such as maternal weight, diabetes, smoking, ethnicity and twins. Wrong gestational dating is a common cause of an abnormal MoM.
- A: Non-pregnant ranges do not apply to pregnancy markers.
- C: Control results measure assay precision, not the patient population.
- D: A mean across all stages ignores the large weekly change in marker levels.
Question 6 · Easy
When does maternal serum hCG reach its highest level in a normal singleton pregnancy?
1. A8 to 11 weeks of gestation
2. B36 to 40 weeks of gestation
3. C16 to 20 weeks of gestation
4. D3 to 4 weeks of gestation
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Question 7 · Hard
A woman at 10 weeks with a live intrauterine pregnancy on ultrasound has a negative qualitative urine hCG test on a concentrated first-morning sample. Her serum hCG on the laboratory analyzer is 210,000 IU/L. What best explains the urine result?
1. AHook effect from excess hCG and beta-core fragment
2. BHeterophile antibodies in the urine
3. CUrine too dilute to detect hCG
4. DCross-reaction with luteinizing hormone
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Question 8 · Easy
Which fetal tissues produce most of the alpha-fetoprotein (AFP) that reaches the mother's blood?
1. AFetal kidneys and bladder
2. BFetal adrenal cortex
3. CPlacental syncytiotrophoblast
4. DYolk sac and fetal liver
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Question 9 · Easy
During pregnancy, human chorionic gonadotropin (hCG) is made mainly by the:
1. ASyncytiotrophoblast of the placenta
2. BFetal liver and yolk sac
3. CCorpus luteum of the ovary
4. DAnterior pituitary gland
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Question 10 · Medium
A woman with lower abdominal pain at an estimated 6 weeks of pregnancy has the serial serum hCG results shown. No intrauterine pregnancy is seen on ultrasound. The pattern is most consistent with:
Figure: MedExams
1. AA molar pregnancy
2. BAn ectopic or non-viable pregnancy
3. CA complete miscarriage
4. DA normal intrauterine pregnancy
Answer and explanation in LearnFree account

Pregnancy and prenatal screening

Why are maternal serum screening results adjusted for the mother's weight?

At 12 weeks, a 38-year-old woman with a singleton pregnancy has first-trimester combined screening: nuchal translucency 3.6 mm (above the 99th centile), free beta-hCG 2.4 MoM and PAPP-A 0.35 MoM. These results most increase the risk of:

At 17 weeks, dated by ultrasound, a woman carrying one fetus has maternal serum AFP 3.2 MoM (screen-positive cut-off 2.5 MoM). Her hCG, unconjugated estriol and inhibin A are all near 1.0 MoM. Which condition does this result most suggest?

Prenatal screening results are reported as multiples of the median (MoM). A MoM is the patient's result divided by the:

When does maternal serum hCG reach its highest level in a normal singleton pregnancy?

A woman at 10 weeks with a live intrauterine pregnancy on ultrasound has a negative qualitative urine hCG test on a concentrated first-morning sample. Her serum hCG on the laboratory analyzer is 210,000 IU/L. What best explains the urine result?

Which fetal tissues produce most of the alpha-fetoprotein (AFP) that reaches the mother's blood?

During pregnancy, human chorionic gonadotropin (hCG) is made mainly by the:

A woman with lower abdominal pain at an estimated 6 weeks of pregnancy has the serial serum hCG results shown. No intrauterine pregnancy is seen on ultrasound. The pattern is most consistent with: