Which scans or tests show up any abnormalities?

You’ll be offered a range of prenatal tests designed to check that you baby is developing normally.

Some of these are screening tests and only assess your risk factor – they don’t provide a definite diagnosis. Only invasive tests (so called because they effectively ‘invade’ your unborn baby’s space) can provide a diagnosis but carry a small risk of miscarriage. You need to think very carefully before proceeding with an invasive diagnostic test as, in some cases, the risk of miscarriage may actually be higher than your baby’s assessed risk of having a birth defect.

First trimester tests

First Trimester Screen

This screening test is carried out between weeks 11 and 13 and combines a blood test performed on you with what’s called a nuchal translucency scan performed on your baby. This type of scan measures an area of fluid behind your baby’s neck that has been linked with chromosomal disorders such as Down syndrome. It doesn’t check for neural tube defects such as spina bifida.

Its success rate in identifying babies with chromosomal abnormalities is 85 percent – so 85 out of every 100 affected babies will be identified. Its false positive rate is 5 percent. If you get a positive result you’ll be offered a diagnostic test.

Chorionic villus sampling

Often referred to as CVS, this diagnostic test is carried out between weeks 10 and 13. It is an invasive test and involves having a sample of placental tissue removed via a long thin needle inserted through your abdomen under ultrasound guidance. It can also be carried out with a catheter inserted through the cervix.

It checks your baby for chromosomal disorders and genetic disorders, such as cystic fibrosis. Its success rate is high – 98-99 percent – though it will not indicate how severely a baby is affected.

If your First Trimester Screen assesses your baby as high risk for chromosomal disorders, you’ll likely be offered a CVS for diagnosis. It is considered safe but it is an invasive test and carries a 1 percent risk of miscarriage.

Second trimester tests

Blood tests

These screening tests are carried out between weeks 15 and 20 and include the alpha-fetoprotein test, the triple screen test and the quad screen. Each checks for various ‘markers’ in your blood that are signs your baby may have a chromosomal or neural tube disorder.

They identify 75-90 percent of babies with neural tube defects and 60-80 percent of babies with Down syndrome, so they have a fairly high rate of false positive results. A false positive result may occur if you’re pregnant with twins or more, or your pregnancy is further along than you thought as both can result in high levels of the markers these tests check for.

Second trimester scan

This screening test checks your baby’s physical development and is carried out between weeks 16 and 20. It can detect signs of spina bifida, heart abnormalities and other birth defects, such as cleft lip and palate.

It can be up to around 80 percent accurate if carried out by an experienced sonographer using state-of-the-art equipment; however, the American College of Obstetricians and Gynecologists recommends scans be given only when medically necessary, so you may not be offered one if you’re low risk.

Amniocentesis

This diagnostic test is carried out between 14 and 20 weeks. Under ultrasound guidance, a long thin needle is inserted through your abdomen and into your uterus to take a sample of the amniotic fluid. This contains cells from your baby which are tested for chromosomal disorders, neural tube defects and genetic disorders.

Amniocentesis has a 99-100 percent accuracy rate but doesn’t indicate how severely a baby may be affected. As an invasive test, it carries a 0.5 percent miscarriage risk. If your blood tests return a positive result you’ll probably be offered amniocentesis for a diagnosis.

Cordocentesis

This diagnostic test is carried out after week 17. Under ultrasound guidance, a thin needle is guided through the abdomen and uterus into the umbilical cord to take a sample of the baby’s blood.

Cordocentesis checks for chromosomal and blood disorders, but not for neural tube defects. The test is almost 100 percent accurate but is invasive and carries a 1-2 percent risk of miscarriage.

Third trimester tests

Non-stress test

Carried out after 28 weeks if there are concerns your baby isn’t moving as much as he should, this involves monitoring his heart rate and movements. You may be offered this test if you go overdue.

Biophysical profile (BPP)

This is carried out after week 32 if there is concern about your health or your baby’s. It involves monitoring your baby’s heart rate over 20-30 minutes and performing a detailed ultrasound scan to check your baby’s activity levels and the amount of amniotic fluid in your uterus.

The information in this feature is intended for educational purposes only. If you have any concerns about your health, the health of your child or the health of someone you know, please consult with a doctor or other healthcare professional.

Related Articles

• Tests & Checks: Nuchal Translucency Test
• Tests & Checks: Anomaly scan
• Early Pregnancy: Ultrasound scans
• Guide To : Pregnancy: Antenatal care and Ultrasound scans
• Tests & Checks: Dating Scan