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

Flowchart 2.2   Antenatal screening

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