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 who have a history of a previous child or fetus with Down syndrome, trisomy 18 or trisomy 13;
• Women who are HIV positive; and
• Women pregnant following in vitro fertilization with intracytoplasmic sperm injection (IVF with ICSI).

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 East Kootenay/Kootenay/ Boundary regions 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, women who are HIV positive, and women with a pregnancy conceived by IVF with ICSI.

If a woman does not qualify for IPS (for eg. she is under age 35) but wishes to pay privately, is this service available? 

(MSP covered) NT ultrasound is reserved for those women who are at an increased risk for chromosomal aneuploidy. In BC, women age 35 and older are eligible for the NT ultrasound as part of IPS. Women age 34 and younger are eligible for SIPS (2 part serum only) which offers a comparable detection rate and false positive rate for their age group.

In some parts of the province, NT ultrasound measurement is available on a private pay basis. If a woman wishes to pay privately for the NT and have the SIPS done through the public system, the NT measurement can be used in the risk calculation if the NT was measured by a provider who is accredited by the Fetal Medicine Foundation (provincially-recognized quality control organization). In such cases, the provider ordering the prenatal screen should send a copy of the NT ultrasound report to the Prenatal Biochemistry Laboratory at BC Children's and BC Women's at fax number 604-875-3008. Please mark on the Part 1 Prenatal Biochem Lab Req that an NT ultrasound is being done/ planned (see clinical information question #3 on the lab req). The (privately paid) NT ultrasound measurement will then be incorporated with the (publically offered) SIPS result to provide the woman with an IPS result.

If a woman wants to know her risk of Down syndrome in the first trimester, is that an option?

• Yes, First Trimester Screening (PAPP-A + free beta-hCG serum biochemistry + NT ultrasound) is done between 11-13w6d gestation and provides an immediate risk for Down Syndrome. It is not available through the public system.
• First Trimester Screening (FTS) is available on a private pay basis (approx. $500) in some parts of the province (Burnaby, Victoria, Kelowna). FTS is a screening test, measuring different serum analytes, other markers on ultrasound, and using different technology to analyze results. It is NOT the same as ordering Part 1 serum (of IPS or SIPS) and an NT ultrasound through the public system.
• If a woman is looking for a private pay service for First Trimester Screening, consider the following to ensure a reliable result:
• Ensure that the provider who takes the ultrasound measurements is accredited by the Fetal Medicine Foundation (provincially-recognized quality control organization).
• Ensure that the serum biochemistry tests (PAPP-A + free beta-hCG) are performed in a DAP – accredited laboratory or a laboratory that has accreditation by a comparable organization.
• Note that the First Trimester Screen does not screen for open neural tube defects (NTD). The second trimester detailed fetal ultrasound scan is therefore required to properly screen for NTDs.

If my patient has had private pay First Trimester Screening (FTS), should I order IPS/SIPS through the public system?

No. Only one screen should be ordered in the pregnancy. Since the screens measure different markers and use different technology, two results can be contradictory and overall increase the false positive rate.

What should I do if the woman has had private pay FTS but I have already ordered IPS/SIPS through the public system?

The IPS/SIPS should be cancelled. Two contradictory results can be generated for the same pregnancy. This will be confusing for the patient.

What should I do if my patient shows me her private pay FTS result after I receive her IPS/SIPS report?

You and your patient will need to discuss the results of both tests. A referral to Medical Genetics can be helpful to sort out the overall meaning of the two reports for this pregnancy (Vancouver 604-875-2157 and Victoria 250-727-4461).

What is Non-Invasive Prenatal Testing (NIPT) and how does it compare to all the other screening tests?

NIPT is a new non-invasive blood test that measures the quantity of cell-free fetal DNA that is normally found circulating in maternal blood. NIPT can be performed anytime after 10 weeks gestation. It is considered a very accurate, safe, and non-invasive screen which has an increased detection rate for Down syndrome (>99%) and trisomy 18 (>98%) with a lower false positive rate (<0.3%) compared to the other offered blood screening tests. NIPT, therefore, is likely to reduce or even prevent the need for a lot of women to have to undergo the invasive amniocentesis procedure as the next step after a positive IPS/SIPS/Quad screen.  

Effective November 2015, provincial funding is available for particular high-risk pregnancies. NIPT is a funded option for women who meet one of the following criteria:

1. first screen positive on IPS/SIPS/Quad;
2. a pregnancy/family history of trisomy 21/18/13; or 
3. pregnancy with both a screen result and ultrasound markers of aneuploidy which increases the risk to 1/300 or more. 
   

Visit our NIPT page for more information on NIPT, how it compares to amniocentesis, and BC locations offering funded NIPT.

Women who are considered at lower risk for chromosomal aneuploidy (or none of the above eligibility requirements apply) should still continue to be offered IPS or SIPS. If a lower risk woman wants to consider NIPT as a first tier screen instead of doing SIPS/IPS, and is willing to pay, there are many private locations in BC offering self-pay NIPT.  See our NIPT page for self-pay NIPT locations.  

Why has the minimum age for amniocentesis without prior screening been changed from 32 to 35 years old for multiple gestation pregnancies?

Previously, the presence of two fetuses was predicted theoretically to increase the risk of Down syndrome in the pregnancy by 1.67. More recent studies have shown that the prevalence of Down syndrome is only 3% higher in multiples than singletons (Cuckle, 1998).

Why are there different qualifying ages for amniocentesis without prenatal screening in singleton and multiple gestation pregnancies?

Prenatal screening tests for Down syndrome are less sensitive in pregnancies with multiple gestations as compared to singleton pregnancies. Therefore, women with twin / triplet pregnancies who have a higher risk of Down syndrome based on age (≥35 years old) have the option of choosing a definitive test (amniocentesis) to rule out Down syndrome.

What is the rationale for offering biochemistry tests to women less than 35 years old with twin pregnancies who do not have access to NT?

•  Women with twin pregnancies will be offered IPS if available. Women 35 years and older who do not have access to NT will also be offered the option of amniocentesis. Women younger than 35 years who do not have access to NT will be offered SIPS or Quad depending on their gestational age.
• While the best screen for Down syndrome and trisomy 18 for women with twin pregnancies is a screening test that includes NT, biochemistry alone continues to be a viable option for women at lower risk for these conditions. While the detection rates are lower for twin pregnancies than singletons, the detection rates achieved justify offering the test to women who wish screening for Down syndrome (Bush, 2005).

What if twins or triplets have an NT measurement that is discordant – should both twins/all triplets be tested by amniocentesis?

Yes. Given the possibility that the twins or two or three of the triplets can be identical, the finding of an increased NT measurement in one twin/triplet justifies cytogenetic testing of all babies.

Two of my patients, one who had Quad and the other SIPS, 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 screen cut-off is 1:385 (risk for delivering a Down syndrome baby at term) while the SIPS cut-offs is 1:300 and the IPS cut-off is 1:200.

Why are screen results not graphically displayed? How are reports issued?

Prenatal screening tests are being interpreted using a software (Alpha) that does not easily enable graphical display of the screen results. Further, inclusion of a graphical display would make the report longer than a single page.

Why is the trisomy 18 risk not displayed/reported?

The Alpha software does not display/report the risk for Trisomy 18 unless it is above screen cut-off.

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 SIPS, 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 screen cut-off is 1:385 (risk for delivering a Down syndrome baby at term) while the SIPS cut-offs is 1:300 and the IPS cut-off is 1:200.

The report says that my patient had her blood drawn too early. Her last menstrual period dates her at 15 weeks and her ultrasound agrees dating her at 14 weeks 3 days, so her due date doesn’t change. Why does she need to have her blood redrawn? 

Dating for serum screening is preferentially based on ultrasound, as measurement of the biparietal diameter or crown rump length is the most precise method of gestational age assessment on a population basis.

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.