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Second Trimester Screening

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. These include brain abnormalities (e.g. ventriculomegaly, hydrocephalus, absent corpus callosum), facial abnormalities (e.g. cleft lip), heart abnormalities (e.g. Fallot's tetralogy, transposition of great arteries), lung abnormalities (e.g. lung sequestration, congenital pulmonary airway malformation), abdominal abnormalities (e.g. omphalocele, gastroschisis, abdominal cysts), spinal and other skeletal abnormalities (e.g. spina bifida, scoliosis, skeletal dysplasias), kidney abnormalities (e.g. hydronephrosis, echogenic kidneys) and genitalia abnormalities (e.g. ambiguous genitalia).

  • 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, and / or other structural abnormalities. Soft markers include thickened nuchal fold, absent nasal bone, echogenic bowel, choroid plexus cyst, renal pelvic dilatation, echogenic focus of the heart, aberrant right subclavian artery (ARSA), persistent left superior vena cava, shortened humerus or femur, and others.

  • 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 be 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). Together with history, weight, height and blood pressure measurements x 2 on both sides, the Astraia software that the clinic uses allows the calculation of risks of pre-eclampsia less than 32 weeks and less than 36 weeks based on the Fetal Medicine Foundation algorithm. An optional blood test to check for Placental growth factor (PlGF) may further increase the detection rate of pre-eclampsia.

  • Cervical length - The most accurate way to measure this is a vaginal scan with an empty bladder. 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. If the cervical length shortens to < 15 mm despite medication, surgery to stitch up the cervix (or cervical cerclage) before 24 weeks may be necessary.


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