Ultrasound and imaging in obstetrics and gynaecology
Introduction
This domain tests first-trimester dating, the mid-trimester anomaly scan, fetal biometry and Doppler, gynaecological pattern recognition and the reporting systems used in Czech practice.
Obstetric ultrasound
- Dating: crown–rump length at 7–13 weeks is the most accurate (±5–7 days); later dating uses head circumference and femur length.
- Nuchal translucency is measured at CRL 45–84 mm, between 11+0 and 13+6 weeks, in a mid-sagittal plane.
- Anomaly scan at 18–22 weeks: head, spine, cardiac outflow tracts, abdominal wall, kidneys, limbs, placenta and cord insertion.
- Biometry: BPD, HC, AC, FL with estimated fetal weight; AC is the most sensitive marker of growth disturbance.
- Amniotic fluid: deepest vertical pocket 2–8 cm, AFI 5–25 cm.
- Doppler: umbilical artery PI falls with gestation, and absent or reversed end-diastolic flow signals severe placental insufficiency. Middle cerebral artery PI falls with hypoxia; the cerebroplacental ratio detects brain sparing.
- Cervical length under 25 mm before 24 weeks predicts preterm birth and identifies candidates for progesterone or cerclage.
Gynaecological ultrasound
- Simple adnexal cyst: thin wall, anechoic, no septation - follow or discharge according to size and age.
- IOTA simple rules and O-RADS stratify adnexal masses; IETA terminology describes the endometrium.
- Postmenopausal endometrial thickness ≤4 mm has a high negative predictive value for carcinoma.
- BI-RADS is used for breast lesions.
Worked example
A 29-year-old at 32 weeks has an estimated fetal weight on the 6th centile. Umbilical artery PI is above the 95th centile with absent end-diastolic flow, and MCA PI is low with a cerebroplacental ratio below 1. This is growth restriction with placental insufficiency: admit, give corticosteroids, monitor CTG and Doppler at least twice weekly, and deliver if reversed flow or an abnormal CTG appears.
Traps
- Redating a pregnancy after 14 weeks on a late scan.
- Measuring NT outside the 45–84 mm CRL window.
- Calling a small fetus growth restricted without Doppler.
- Reassuring on a solid adnexal mass with papillary projections because CA125 is normal.