What is the benefit of adding a nuchal translucency (NT) ultrasound measurement for women 35 years and older?

Although the serum integrated prenatal screen (SIPS) is associated with a high detection rate for Down syndrome, the combination of serum screening and NT (integrated prenatal screen or IPS) reduces the false positive rate.

Who qualifies for NT as part of IPS?

As of March 2011, Serum Integrated Prenatal Screen (SIPS) is available to all pregnant women. The following women are eligible for NT ultrasound as a component of Integrated Prenatal Screen (IPS = SIPS in combination with NT):

• Women ≥ 35 year old at expected date of delivery (EDD);
• Women with twin pregnancies;
• Women pregnant following in vitro fertilization with intracytoplasmic sperm injection (IVF with ICSI) and without preimplantation genetic testing.
  

In order to ensure the quality of NT ultrasound, every sonographer must annually perform a minimum number. As such, pregnant women 30 years and older from the Northern Health Authority and Kootenay Boundary Region are also eligible for an NT ultrasound as part of IPS (in order for sonographers to perform annually the minimum number of NT's required to maintain certification).

What accommodation has been made in the guidelines for communities with limited/no access to NT?

• There are many NT sites are operational in all of BC's health authorities. Eligible women able to travel may be accommodated at any NT site.
• The risk cut-off for SIPS has been adjusted to enable the same detection rate for Down syndrome in women 35 years and older as IPS (NT and serum integrated biochemistry). False positive rates will be higher for women who have not had an NT.
• Detection and false positive rates for SIPS and Quad screening meet the recommendations of the Society of Obstetricians and Gynaecologists of Canada (SOGC) and the Canadian College of Medical Geneticists (CCMG).

What if a woman qualifies for an NT ultrasound measurement but cannot get it because there is no NT centre nearby that provides this service or the nearby centre is fully booked?

• If the woman is able to travel, she may be accommodated at any NT site in the province; OR
• Offer SIPS. This is a good alternative as it provides an equivalent high detection rate for Down syndrome and the only downside is that the false positive rate is higher than with the integrated prenatal screen (SIPS plus NT ultrasound).
• For women 40 or older with a singleton pregnancy or 35 or older with a multiple gestation pregnancy, amniocentesis should be offered as an option.

Why is IPS not offered to women younger than 35 years old as part of the BC Prenatal Genetic Screening Program?

• SIPS and Quad screens are available to women of all ages and are high quality screens that meet recommended national and international standards for prenatal screening.
• The capacity in BC for NT ultrasound measurement is limited (due to technological and medical expertise) and NT is reserved for women who would benefit most.
• The chance of a false positive result is higher for women 35 and older, thus, women in that age group can benefit most.
• NT is also offered where the woman or her partner has had a previous child or fetus with Down syndrome, trisomy 18 or trisomy 13 (trisomy 13 increases the risk of Down syndrome), women with multiple gestations, and women with a pregnancy conceived by IVF with ICSI.

In practical terms, what does the detection rate of a screening test mean?

The detection rate is the chance that an affected pregnancy will be detected by the screening test. For example, SIPS in women less than 35 years has a detection rate for Down syndrome of 79%. This means that the screen will detect 79% of pregnancies with Down syndrome in women less than 35 years of age. It also means that if a woman less than 35 years of age has a fetus with Down syndrome, there is a 79% chance that she will screen positive.

In practical terms, what does the screen positive rate mean?

• The screen positive rate is the chance that a test will be above the screen cut-off and the result will be reported as screen positive. This rate includes both false positives and true positives. A false positive means that the screen is positive but the baby is unaffected whereas a true positive means that the pregnancy screens positive and the baby is affected. The screen positive rate varies for the different tests and for women of different ages. However, for all tests and all age groups, the majority of women who screen positive have an unaffected pregnancy.
• For example, women less than 35 years of age, who choose SIPS, have a 3.7% chance of having a screen positive result. Of those women who screen positive, 97% of them will have an unaffected pregnancy. These women have a false positive screen result.

In practical terms, once a woman has a positive screen result, should she be counselled as per the risk estimate on the report?

• Yes. The risk estimate provided on her report is the best estimate of her risk for having an affected pregnancy based on the combination of her age, the results of her blood test(s) and, if available, the nuchal translucency measurement. The patient’s calculated risk as indicated on her report should be used in the counselling.
• For example, the 32 year old woman who had SIPS and screens positive with a risk of Down syndrome of 1 in 240 should be counselled that the best estimate of her risk is 1 in 240. This means that for 240 women with this same risk, one will have an affected baby and 239 will not.

Two of my patients, one who had Quad and the other IPS, had the same Down syndrome risk but one was called a screen negative and the other was called a screen positive. How can this be? 

The Quad and SIPS screen cut-off is 1:900 (risk for delivering a Down syndrome baby at term) while the IPS cut-off is 1:200.

Should the Prenatal Biochemistry Laboratory at BC Children's and BC Women's be contacted regarding soft markers on the 18-20 week ultrasound?

No. The Prenatal Biochemistry Laboratory will no longer be amending reports based on soft markers.

Should the Prenatal Biochemistry Laboratory be contacted if dating by second trimester ultrasound differs from the original dates calculated by LMP?

Yes, if dating by second trimester ultrasound differs by more than seven days from original dates, fax the ultrasound report to the Prenatal Biochemistry Laboratory (fax 604-876-3008) for possible recalculation of risk. The only exception would be when a screen result is positive for trisomy 18. In these cases, the screen will not be recalculated (because trisomy 18 is frequently associated with intrauterine growth restriction).

Does a family history of Down syndrome in a relative other than the woman/partner’s child increase the risk of having a baby with Down syndrome? 

If the karyotype of the affected family member (other than child) is 47,XX, +21 or 47, XY, +21, the woman is not at increased risk of having a pregnancy with Down syndrome.

Why is smoking status requested for women having a serum screen?

• The maternal serum level of inhibin-A is significantly influenced by any maternal smoking.
• Providing this information on the patient requisition will allow for adjustments to the risk calculation to be made, improving the accuracy of the screen result.

Is the risk assessment adjusted for IVF pregnancies?

• The literature consensus is that biochemistry results differ slightly in IVF pregnancies when compared to non-IVF pregnancies leading to relatively higher screen positive rates in IVF pregnancies.
• In order to prevent this effect, IVF specific adjustment factors are applied to biochemistry results from IVF pregnancies to determine the Down syndrome risk.

Why is the patient’s ethnicity needed for the risk assessment? 

Each biochemical marker shows slight ethnic differences. A woman’s marker levels are compared to other women of the same ethnicity as part of the risk calculation. If the ethnic group is not indicated, the calculation will be less accurate as marker levels will be compared to the overall population of women being screened, regardless of ethnic group.