The NHS 20-week scan: a milestone for UK parents
The mid-pregnancy scan offered on the NHS between 18 and 21 weeks, widely known simply as the "20-week scan", checks the baby's physical development in detail and is often the appointment where parents choose to find out the baby's sex, if they wish. It is offered to every pregnant woman in the UK as a matter of routine, unlike some countries where a comparable scan requires a specific referral.
Crown-rump length versus head-to-heel length
Early in pregnancy, sonographers measure "crown-rump length" (from the top of the head to the bottom of the torso) because the legs are curled up and hard to measure accurately. From around week 20 onward, a full head-to-heel length becomes the standard measurement, which is why the length figures in guides like this one jump noticeably around that point rather than increasing smoothly.