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Wolfson Institute of Preventive Medicine - Barts and The London

Calculating the risks of Down syndrome, Edwards syndrome and Patau syndrome

This page provides information about calculating the risk or chance of a pregnancy with Down syndrome, Edwards syndrome or Patau syndrome.

What is meant by ‘risk’ or 'chance'?

A risk or chance is how likely an event will occur. For example, a risk or chance of Down syndrome of 1 in 100 means that if 100 women have this test result, we would expect that 1 of these women would have a baby with Down syndrome and that 99 would not. This is the same as a 1% chance that the baby has Down syndrome and a 99% chance that the baby does not.

When calculating the risk or chance of Down syndrome, Edwards syndrome or Patau syndrome one should take the following into consideration:

Maternal age

The risk or chance of having a baby born with Down syndrome increases with maternal age as shown in the table below. It is known as the maternal age-specific risk or chance and is the background risk or chance of Down syndrome used when interpreting a screening result.  As maternal age increases the risk or chance of having a baby with Edwards syndrome or Patau syndrome also increases.  However these two conditions are much less common than Down syndrome.

Maternal age at term Risk of Down syndrome Maternal age at term Risk of Down syndrome Maternal age at term Risk of Down syndrome
20 1:1450 30 1:940 40 1:85
21 1:1450 31 1:820 41 1:70
22 1:1450 32 1:700 42 1:55
23 1:1400 33 1:570 43 1:45
24 1:1400 34 1:460 44 1:40
25 1:1350 35 1:350 45 1:35
26 1:1300 36 1:270 46 1:30
27 1:1200 37 1:200 47 1:30
28 1:1150 38 1:150 48 1:30
29 1:1050 39 1:110 49 1:25
Morris et al (2003)

Effect of maternal age on screening performance

An older woman is more likely to have a screen-positive (high risk or chance) result than a younger woman as she starts with a higher age-specific risk or chance of Down syndrome, Edwards syndrome or Patau syndrome. For this reason, the test is more likely to detect a pregnancy with Down syndrome, Edwards syndrome or Patau syndrome in an older woman than in a younger woman. Whatever the woman's age, the best estimate of her risk or chance is obtained using her age in conjunction with her marker values.

Compare the age specific performance of the three screening tests:

 

Markers

There are several different markers that are used in screening for Down syndrome, Edwards syndrome and Patau syndrome. Some are useful only in the first trimester of pregnancy (11-13 weeks) and some are useful only in the second trimester of pregnancy (14-22 weeks). The three screening tests use different combinations of these markers.

First trimester markers

  • Pregnancy Associated Plasma Protein-A (PAPP-A)
  • free ß-human chorionic gonadotrophin ( free ß-hCG)
  • Nuchal translucency (NT)

Second trimester markers

  • Alpha-fetoprotein (AFP)
  • unconjugated oestriol (uE3)
  • total human chorionic gonadotrophin (hCG)
  • Inhibin-A (inhibin)

In the first trimester of pregnancy the PAPP-A level is, on average, lower than expected in pregnancies with Down syndrome, Edwards syndrome and Patau syndrome.  The nuchal translucency measurement tends to be higher than expected for all three conditions.  For free ß-hCG levels are, on average, higher than expected in pregnancies affected with Down syndrome while lower than expected for pregnancies affected with Edwards syndrome and Patau syndrome. for pregnancies.

In the second trimester AFP and uE3 levels are, on average, lower than expected in pregnancies with Down syndrome and Edwards syndrome. The inhibin and hCG levels are, on average, higher in pregnancies affected with Down syndrome and lower in pregnancies affected with Edwards syndrome. The Quadruple test does not screen for Patau syndrome.

MoM

The concentrations of the markers vary with gestational age. In the first trimester PAPP-A and NT increase, while free ß-hCG decreases. In the second trimester AFP and uE3 increase, hCG decreases, and inhibin decreases before 17 weeks and increases after 17 weeks. Also, the measurement of serum markers may vary between laboratories. In order to take account of this variation, the concentration of each marker is expressed as a multiple of the median for unaffected pregnancies of the same gestational age (MoM).

In the diagram below the median marker level is 2.5 iu/mL at 10 weeks, 5.0 iu/mL at 12 weeks and 10.0 iu/mL at 14 weeks. If a woman is found to have 5.0 iu/mL at 10 weeks her level is twice the median (5.0/2.5) or 2.0 MoM. Similarly if the level is 5.0 iu/mL at 14 weeks this is half the median (5.0/10.0) or 0.5 MoM.

Risk of Down syndrome in relation to marker levels

Risk of Down’s syndrome in relation to marker levels

The graphs below show the overlapping relative frequency distributions of the markers in pregnancies with Down syndrome. The points of intersection are the value at which the risk or chance of a pregnancy with Down syndrome is the same as the background risk or chance in the population. From these graphs, it can be seen that AFP, uE3 and PAPP-A values below 0.86 MoM, 0.83 MoM and 0.64 MoM respectively and NT, inhibin-A and free ß-hCG values above 1.46 MoM, 1.54 MoM and 1.67 MoM respectively will tend to increase the risk or chance of a pregnancy with Down syndrome above the background risk or chance while values in the opposite directions will tend to decrease the risk or chance below the background risk or chance

For a 75K PDF showing all six curves more clearly, please click any graph above.

Factors affecting the screening test

Maternal weight, ethnic group, In Vitro Fertilisation (IVF), Insulin Dependent Diabetes Mellitus (IDDM) and smoking

  • Serum marker levels tend to be decreased in heavier women, and increased in lighter women.
  • AFP levels tend to be about 20% higher, free ß-hCG and hCG levels about 10% higher and PAPP-A levels about 60% higher in Afro-Caribbean women than in Caucasian women.
  • Free ß-hCG and hCG levels tend to be about 10% higher and uE3 and PAPP-A levels about 10% lower in women who have become pregnant as a result of IVF compared with non-IVF pregnancies
  • AFP and uE3 levels tend to be low (about 8% and 6% respectively) in women with insulin dependent diabetes mellitus.
  • PAPP-A, free ß-hCG and hCG levels tend to be about 20% lower and inhibin levels about 60% higher in women who smoke.

Appropriate adjustments of the MoM values are made for these factors.

Twins

The serum marker levels are raised in twin pregnancies. Adjustments are made to take account of this.

Screening in twin pregnancies poses a difficulty because of the possibility that one fetus may be affected and the other may not. Because of the presence of two fetuses the amniocentesis is a slightly more complex procedure in a twin pregnancy. If one twin is found to be affected and the other unaffected, selective feticide can be offered.

Previous affected pregnancies

If a previous pregnancy with Down syndrome, Edwards syndrome or Patau syndrome  is reported, the result may be classified as ‘screen-positive’ (high risk or chance) regardless of the level of the screening markers so that further testing can be discussed with the woman. A risk or chance is calculated which takes account of a woman’s previous pregnancy with Down syndrome. The woman's age at the time of her previous pregnancy with Down syndrome affects the risk or chance of a subsequent pregnancy being affected with Down syndromeand this is taken into account in the risk or chance calculation.

Taking account of screening in a previous pregnancy

If a woman has been screened in a previous pregnancy the levels of the screening markers in that pregnancy can be used to adjust the marker levels in the current pregnancy. This is useful because markers used in screening tend to 'track' between pregnancy (e.g. a free ß-hCG level that is high in one pregnancy tends to be high in a subsequent pregnancy). So a woman with a false positive (high risk or chance) result in one pregnancy is likely to have a false positive (high risk or chance) result again in a subsequent pregnancy. Adjusting marker levels for those in a previous pregnancy can help avoid this problem of false-positive (high risk or chance) results recurring in different pregnancies.

Vaginal bleeding

Vaginal bleeding prior to taking a blood sample for the quadruple test can affect the screening result by increasing the maternal serum AFP level and so, in these circumstances, it may be advisable to delay collecting blood for the screening test until a week after bleeding has stopped.

Testing after amniocentesis

If an amniocentesis has been attempted in the pregnancy prior to taking the second blood sample, the result cannot be interpreted. This is due to the possibility of feto-maternal transfusion which can increase the maternal serum AFP level.

Demised Twin

If, when an ultrasound scan is performed, there is the presence of a demised twin then the serum markers should not be used in the screening interpretation as the markers maybe adversely affected – for example the AFP levels can be substantially higher than expected when a demised twin is present.  Use of the serum markers can only take place once the demised twin has disappeared – a pregnancy where there is an empty gestational sac can have the serum markers measured.

Multiple pregnancies (triplets or more)

If the pregnancy has three or more foetuses present the serum markers cannot be used in the screening interpretation.