NON INVASIVE PRENATAL TESTING (NIPT)
What is NIPT?
NIPT is a recent advance in prenatal screening, which analyses cell free fetal DNA circulating in the maternal blood.
It is done on the mother’s blood sample which contains fetal DNA and helps to screen for the most common fetal chromosomal abnormalities like Down’s syndrome (Trisomy 21), Edward’s syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13).
What is cell free fetal DNA?
DNA from the fetus circulates in maternal blood. Unlike intact fetal cells in maternal blood, which can persist for years after a pregnancy, circulating cell-free fetal DNA results from the breakdown of fetal cells (mostly placental) and clears from the maternal system within hours. Fetal DNA detected during a pregnancy, therefore, represents DNA from the current fetus.
Why should this test be done?
Prenatal (during pregnancy) diagnosis employs a variety of techniques to determine the health and condition of an unborn fetus. Without knowledge gained by prenatal diagnosis, there could be an untoward outcome for the fetus or the mother or both. Congenital anomalies account for 20 to 25% of perinatal deaths. The most well known and most common autosomal abnormality is Down syndrome, with an incidence of 1 in 1000 births. Every year about 3000 to 5000 babies are born with this disorder.
Prenatal diagnosis is helpful for:
• Managing the remaining weeks of the pregnancy
• Determining the outcome of the pregnancy
• Planning for possible complications with the birth process
• Planning for problems that may occur in the newborn infant
• Deciding whether to continue the pregnancy
• Finding conditions that may affect future pregnancies
Who should be tested?
It can be performed in all pregnancies without any specific indication.
However, it is most appropriate in pregnancies which have an increased risk for fetal chromosomal abnormalities based on maternal age (>35 yrs), previous history of child with chromosomal abnormality or abnormal result of a triple or double marker test.
It can be used as an alternative to amniocentesis or chorionic villus sampling.
When can the test be done?
The test can be performed any time after the 10th week of gestation (2.5 months after last date of missed period)
What are the advantages of this test?
• Non-invasive and hence there is no risk to the fetus
• High sensitivity (>99%) for detection of Down’s syndrome, ié percentage of fetus who are correctly identified as having abnormality is high.
• Low false positive rate for detection of Down’s Syndrome ié percentage of fetus who are incorrectly identified as having abnormality is low.
Does this test detect all chromosomal abnormalities?
NO. It helps to screen for the most common fetal chromosomal abnormalities like Down’s syndrome (Trisomy 21), Edward’s syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13). There are many other abnormalities like molar pregnancy, neural tube defects, monosomy, Triploidy, congenital infections, which are not detected by this test.
What is the turn around time for the test results?
2- 3 weeks
What does the test result mean?
A HIGH RISK result indicates a high risk for a trisomy, it does not mean the fetus definitely has a trisomy.
A LOW RISK means it is unlikely that the fetus has a trisomy, but it cannot be excluded.
Follow up of test results
➢ In case of a normal NIPT result: no specific follow up is necessary unless ultrasound examination of the fetus reveals anomalies and further fetal studies might be indicated.
➢ In case of test failure: in a limited number of pregnancies (< 4%) not enough fetal DNA can be extracted from the maternal blood, and NIPT cannot be performed. This has no implications on the risk of fetal aneuploidies or other fetal anomalies. In these pregnancies NIPT can be done on a repeat maternal blood sample.
➢ In case of an abnormal NIPT result: in case of an abnormal result, the physician or genetic counsellor will discuss the implications of such chromosomal anomaly with the patient, who can then decide to confirm the NIPT results with chromosome studies after amniocentesis or chorion biopsy.