In this Section:
Integrated Prenatal Screening (IPS)
The Society of Gynecology and Obstetrics of Canada recommends that “all pregnant women in Canada, regardless of age, should be offered, through an informed consent process, a prenatal screening test for the most common clinically significant fetal aneuploidies in addition to a second trimester ultrasound for dating, growth, and anomalies.” The commonest method of fulfilling this is to do Integrated Prenatal Screening (IPS) testing combined with an 18-20 week ultrasound.
Integrated Prenatal screening is a three step process.
- Nuchal translucency (NT) and early anatomy Ultrasound. This is done at 11-13 weeks, but ideally should be scheduled between 11 weeks 5 days and 13 weeks 6 days. The gestational age is confirmed. The tissue at the back of the fetus’s neck (NT) is measured. (Babies with trisomy 21 and other aneuploidys tend to accumulate more fluid here)
- The first blood test must be done on the same day. Your patient must have the correct paperwork with them to complete the test. This test measures proteins in the blood and is used to modify the risk rating.
- The second blood test is done between 15 – 18 weeks.
Results are labeled as either screen positive or negative.
- Screen Positive - If the results indicate a greater than 1/200 risk of Down’s syndrome or a 1/100 risk of Trisomy 18 or 13 they are considered positive. 2% of all tests will be positive. Most will be false positive. Close to 90 per cent of the pregnancies affected with Down syndrome will screen positive (true positive).
- Screen Negative - A screen negative result indicates the chance of Down syndrome is less than 1/200. More than 99 % of pregnancies with normal 21, 18 and 13 chromosomes will screen negative. Please remember that 10% of pregnancies with Down syndrome will also screen negative (false negative).
- Placental Function -
The same biochemical factors that are used in IPS testing have been found to reflect placental function and can identity a group of patients at risk for placental insufficiency. These patients have higher rates of IUGR, SGA, premature delivery and fetal demise. This group of patients can be further stratified by examining the maternal uterine arteries.
The IPS is considered to be false positive when both aneuploidy (trisomy 21, 18 and 13) and neural tube defects are excluded. Adverse pregnancy outcomes have been associated with false positive IPS with the following abnormal maternal serum markers:
1st trimester 10-13 wks bloodwork: 2nd trimester 15-20 wks bloodwork:
low papp-a (<0.35 MOM) AFP > 2.0 MOM
hCG >5.0 MOM
inhibin A >3.0 MOM
In the setting of a false positive IPS described above, measurement of maternal uterine artery Doppler has been shown to identify a group of pregnant women with the highest risk of adverse pregnancy outcomes. Uterine artery Doppler is typically performed between 18-22 weeks and an abnormal Doppler measurement with the maternal uterine artery Pulsatility Index (PI) > 1.45 suggests there is high risk for subsequent placental insufficiency. This patient will require early referral to an obstetrician. A normal PI is reassuring but should be followed with a third trimester ultrasound for fetal growth.
We ask that if you receive an abnormal IPS result, proceed with the 18-20 week ultrasound already booked, refer to genetic counseling for further testing, and refer the patient for uterine artery Doppler (18-22 weeks). In Barrie, this test is currently offered at the RVH but services will expand to the local clinics if demand is high. Please fax the IPS results with the ultrasound requisition.
Uterine Artery Doppler should also be considered in women at risk of placental pathology with current obstetric risk factors (pre-existing hypertension, gestational hypertension, thrombophilia, pre-existing renal disease and Type 1 Diabetes with vascular complications) as well as past obstetric risk factors (early onset gestational hypertension, placental abruption, IUGR and stillbirth).
Uterine Doppler will not be done in low risk pregnancies as it is not proven as a useful tool.
- Twin Pregnancy - If your patient is having twins, the chorionicity can be most accurately assessed at an early ultrasound. This will affect your patient’s management throughout the pregnancy. Identical twins are at higher risk of complications. Monochorionic/monoamniotic and monochorionic/diamniotic pregnancies need early referral to obstetrics. Blood tests are not accurate in twins or triplets so the best screening test is the nuchal translucency which is unique to each baby.
Useful linksThe Society of Obstetrics and Gynecology of Canada 2007 Guidelines on Screening for aneuploidy
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