Source- Antenatal care, NICE guideline, Published: 19 August 2021

First Trimester-

1. Offer ultrasound between 11+2 weeks and 14+1 weeks( CRL between 45 mm to 84 mm)  to:

• determine gestational age
• detect multiple pregnancy

(  if opted for) screen for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome 

Second Trimester- Between 18 +0 and 20+ 6 weeks to

• screen for fetal anomalies (see the NHS fetal anomaly screening programme)
• determine placental location.

The scan can be completed up to 23 weeks + 0 days (23+0) for women in the following three circumstances.

• For women who start screening between 18+0 and 20+6 and require a single further scan to complete screening where the image quality of the first scan is compromised by increased maternal body mass index (BMI), uterine fibroids, abdominal scarring or sub-optimal fetal position.
• Where providers can arrange the fetal anomaly scan later.  The screening pathway must be completed by 23+0.
• Women who present to service at ≥ 20+6 where the sonography department can offer a screening scan appointment and complete screening by 23+0.

Ultrasound later in pregnancy-

• If there are concerns that the symphysis fundal height is large for gestational age, consider an ultrasound scan for fetal growth and wellbeing.
• If there are concerns that the symphysis fundal height is small for gestational age, offer an ultrasound scan for fetal growth and wellbeing, the urgency of which may depend on additional clinical findings, for example, reduced fetal movements or raised maternal blood pressure. 
•  Do not routinely offer ultrasound scans after 28 weeks for uncomplicated singleton pregnancies.
• – If breech presentation is suspected on abdominal palpation, offer an ultrasound scan to determine the presentation.
• In unexplained vaginal bleeding, offer to carry out placental localisation by ultrasound if the placental site is not known.

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