Antenatal genetic and ultrasound tests for baby

• There is a small chance in every pregnancy that baby may have an anomaly. Some women won't want to know, other women are very anxious about possible problems
• Screening for and diagnosis of anomalies gives women the option to prepare for a baby with this type of condition or terminate pregnancy
• Anomalies detected by routine screening before the baby is born include
• Chromosomal — Trisomy 21 (Down syndrome), Trisomy 18
• Structural and functional anomalies

• If results indicate any concerns — medical consult and consider referral to obstetrician

Increased likelihood of foetal anomaly if

• Inherited conditions in woman’s or partner’s family
• Mother has medical problems — diabetes, epilepsy, prescribed medicines, substance misuse
• Previous baby with an anomaly
• Increasing age of mother increases chance of some anomalies (eg Down syndrome)

Screening tests

• Estimate chance that baby may have an anomaly
• Are gestation specific (done at specific times during pregnancy) so accurate dating of pregnancy by ultrasound is needed

Do

• Offer antenatal screening — see Flowchart 2.2
• Talk about testing for chromosomal anomalies at first antenatal visit — use Menzies 'Checking for problems with the baby' resource
• If increased likelihood of foetal anomaly
• Midwife/obstetrician consult to talk about screening and options
• If specific concerns due to age, personal or family history — medical consult
• If a screening test is returned with an anomaly more tests will be needed for diagnosis

First trimester — between 9 and 13⁺⁶ weeks of pregnancy

Maternal serum screen and nuchal translucency measurement

• Screens for
• Down syndrome — detects 80–90% of affected babies
• Trisomy 18
• Take blood from woman between 9 and 13⁺⁶ weeks of pregnancy
• Ultrasound between 11 and 13⁺⁶ weeks of pregnancy to measure back of baby’s neck 

Second trimester screen — between 14 and 20 weeks of pregnancy

Maternal serum screen

• Screens for
• Down syndrome — detects 70–80% of affected babies
• Trisomy 18
• Neural tube defect — anencephaly, spina bifida
• Take blood from woman after 14 weeks and up to 20 weeks pregnant
• Best done around 16 weeks to allow time for diagnostic testing if needed

Tests for foetal growth and anatomical anomalies

First trimester/dating scan

• Most accurate for dating between 8 and 13⁺⁶ weeks pregnant
• Reliably diagnoses multiple pregnancy
• Confirms pregnancy is intrauterine and helps exclude ectopic pregnancy
• Confirms pregnancy is viable — foetal heart activity can be seen on transvaginal ultrasound at 6–7 weeks in normal pregnancy
• Detects some severe structural anomalies, eg anencephaly (parts of brain and skull not formed)
• Can diagnose miscarriage

Obstetric morphology ultrasound

• Usually done at 18–20 weeks pregnant
• Provides information about location of placenta, amount of amniotic fluid, growth of baby
• Reliably detects some major anatomical anomalies (eg open neural tube defects), but less sensitivity for others (eg heart anomalies)
• Detects other anomalies that may not have functional significance but can be associated with chromosomal problems
• If any anomaly — medical consult. Obstetric consult usually needed

Late second trimester and third trimester ultrasound

• If morphology scan not done at 18-20 weeks pregnant — still worth doing ultrasound later
• Not as accurate for dating and detecting anatomical anomalies
• If woman had morphology ultrasound — further ultrasounds only needed if clinically indicated

Diagnostic tests for chromosomal anomalies

• Include chorionic villus sampling and amniocentesis — must be done by a specialist
• Full results from diagnostic testing can take 2-3 weeks
• Preliminary result may be available in 48-72 hours but must talk with obstetrician about results
• Small increase in risk of miscarriage
• If result abnormal — woman may choose termination of pregnancy
• Amniocentesis — needle passed through wall of uterus into amniotic fluid. Cells from aspirated fluid tested
• Done after 15 weeks pregnant
• Can be done at smaller centres
• Chorionic villus sampling (CVS) — needle passed through wall of uterus into placenta to collect cell sample
• Done after 11 weeks pregnant
• Only done in larger hospitals, eg Adelaide

Supporting resources

• Screening for foetal abnormalities and diagnosis poster
• Checking for problems with baby in early pregnancy booklet