What is prenatal diagnosis?
Prenatal diagnosis uses a variety of testing methods to determine whether a fetus has certain genetic or physical abnormalities before birth. At different stages during pregnancy, these methods are used to detect issues such as Down syndrome (trisomy 21), hemophilia, thalassemia, Turner syndrome, spina bifida and Huntington’s disease.
The International Society for Prenatal Diagnosis (ISPD) was founded in 1996 to advance the “science and practice of genetics and fetal care worldwide [and] promote the health of children, their mothers and their families.“[^1]
What is prenatal screening?
Prenatal screening tests can determine the likelihood of a fetus having an abnormality. Parents at particular risk of certain problems may choose to use a variety of prenatal diagnostic tests. This decision is based on factors including their level of risk, the prenatal (before birth) or postnatal (after birth) care required to address a particular issue, and the parents’ comfort with their risk level.
Prenatal screening consists of non-invasive tests, including:
- Maternal serum tests
- Prenatal cell-free DNA (cfDNA) screening (to determine the likelihood that a fetus will have a genetic disorder)
These screenings are considered together to determine a statistical probability of an issue occurring, but they cannot diagnose the existence of these issues with certainty. If a fetus is determined to have a high likelihood of a genetic abnormality, parents are encouraged to undergo diagnostic testing such as amniocentesis and chorionic villus sampling (CVS), which directly test the fetal cells. The amniocentesis and chorionic villus sampling (CVS) tests are more definitive than non-invasive screenings, but do carry risks to the fetus, such as miscarriage or congenital abnormalities.**
Purposes of prenatal diagnosis
In testing for these potential issues, prenatal diagnosis provides information to parents and doctors as early as possible, so that they can determine the best course of care for the fetus, both before and after birth. It can also detect issues that, if left unaddressed, could result in negative outcomes (including death) for the fetus or mother. This knowledge can lessen the risk for conditions requiring prenatal care or that may result in more difficult births.
Additionally, this can tell the doctors whether they should prepare to administer special care to the infant right after birth. It can also allow parents to prepare physically, emotionally and/or financially for the chance that their child will have specific medical needs. Some parents may choose to terminate the pregnancy if the information discovered during prenatal testing suggests that the fetus will not develop into a healthy baby, would not survive long outside the womb or would have an extremely poor quality of life.[^2]
Prenatal diagnostic tests
Amniocentesis is an invasive prenatal diagnostic test in which a long, thin needle is passed through the mother’s abdomen (belly) and uterus into the amniotic sac to extract amniotic fluid. The fetal cells in the amniotic fluid are analyzed for irregularities in chromosomes and biochemicals. This test is usually performed in the second trimester at between 14-20 weeks of pregnancy, or as early as amniotic fluid levels will permit testing.
Amniocentesis can also be used to test for:
- Rhesus factor: This is a very important blood factor which can cause problems when the baby’s Rh factor is different to that of the pregnant person. This can cause Rh sensitization if blood passes between the developing fetus and the mother.
- Fetal lung development (by the third trimester)
Women may experience discomfort and cramping during this test, and should rest for the remainder of the day after it is administered. In rarer cases they may also be advised to rest for 24 or even up to 48 hours after the procedure.
Amniocentesis can detect chromosomal disorders such as Down syndrome; genetic disorders such as cystic fibrosis; and neural tube defects such as spina bifida.[^3] It is not used to detect structural birth defects. Due to its ability to also determine a baby’s gender, amniocentesis is subject to legal restrictions in some countries.
The fetus is not injured during amniocentesis, though the procedure does slightly increase the risk of miscarriage.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) is typically carried out by passing a catheter, guided by ultrasound, from the vagina through the cervix and into the uterus, accessing the chorionic villi cells of the placenta. These cells share genetic material with the fetus. Depending on the placement of the placenta in the mother’s body, it may also be administered transabdominally (through the belly), using a long thin needle, in a procedure similar to amniocentesis.
These cells are then analyzed using a variety of methods, with the most common being chromosomal analysis. CVS detects almost all chromosomal disorders, such as Down syndrome, as well as hundreds of genetic disorders, such as sickle cell disease, cystic fibrosis and Tay-Sachs disease. It does not detect neural tube defects. Unclear results showing a discrepancy between the chromosomes of the cells in the placenta and the cells in the baby, called placental mosaicism, occur between 1-2% of the time with CVS. Amniocentesis (another test to check for chromosomal conditions) may be necessary to gain definitive results.[^4]
CVS can be administered as early as 10 weeks into the pregnancy, in the first trimester. Mothers do not typically experience pronounced discomfort during the test, but some may have light bleeding (spotting) afterwards.
CVS is considered an invasive test, and carries a slightly higher risk of miscarriage than does amniocentesis, as well as other possible risks, including infection or Rh sensitization (in which the Rh positive blood cells from the baby enter the pregnant person’s bloodstream, potentially causing pregnancy complications). In rare cases, it is linked to fetal deformities of the fingers and toes, but current research suggests that this risk is smaller if the test is done after the 11th week of pregnancy.