Before 14 weeks, ultrasound is use for dating purposes.
Dating becomes inaccurate after 14 weeks.
A fetal abnormality scan is done at 20 weeks. This will pick up deformities such as ancephaly ( absence of skull/brain), cleft lip or club foot ( congenital talipes equinovarus). Polyhydramnios (excessive amniotic fluid) can also be picked up during this stage.
The ultrasound can also measure for nuchal tranluceny from 11 to 14 weeks as a screening test for Down's syndrome.