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Second Trimester Ultrasound Screening Scan

Second Trimester Screening Scan or Fetal Anomaly Scan at 18-22 weeks

This scan typically takes 30 minutes but may take longer if the baby is not "co-operative" during the scan. Allow 1 hr or more for this appointment. The following details are obtained during the scan:

  • Structural survey and measurements - This is a systematic screen of major structural abnormalities that can be detected with ultrasound scan, and a set of measurements based on a checklist. This would allow the majority of, but not all, structural abnormalities to be detected.

  • Soft markers - These are often transient ultrasound markers or minor structural variations that when present could increase the risk of the fetus having Down syndrome or other chromosomal abnormalities.

  • Placental location - If the placenta is low-lying at this stage, it is important to check on the placental location again in the third trimester as some of these cases may need to delivered by Caesarean section to prevent severe vaginal bleeding.

  • Uterine arterial Doppler studies - A high resistance on the uterine arterial Doppler studies would predict a higher risk for pre-eclampsia (PE) and intrauterine growth restriction (IUGR). A recently implemented calculator on the Astraia program that the clinic uses allows the calculation of risks of pre-eclampsia less than 32 weeks and less than 36 weeks. An optional blood test to check for Placental growth factor (PlGF) may further increase the detection rate of pre-eclampsia.

  • Cervical length - A long cervical length predicts a very low risk of preterm labour before 33 weeks, while a short cervical length predicts a slightly increased risk of preterm labour. A patient with short cervical length may need to be monitored more frequently for a further shortening of the cervical length and may benefit from daily vaginal pessaries of micronized progesterone 200 mg every night. 


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