Second trimester
20 weeks pregnant: what is happening this week
Week 20 of 42worked example
25.6 cm
- Measured
- head to heelabout the size of a banana
- This week runs
- Thu, May 14, 2026 to Wed, May 20, 2026
- Days to the estimated due date
- 147
- Term window (37-42 weeks)
- Thu, Sep 17, 2026 to Thu, Oct 22, 2026
The NHS publishes this figure for week 20, about the size of a banana. It is an average from patient guidance rather than a measurement of any particular pregnancy, and the measurement basis changes at week 20 - see the note below the dates.
The number leaps this week, from 15.3 cm to 25.6 cm, and nothing dramatic happened overnight. The NHS changes how it measures at week 20: up to here the length is taken head to bottom, because the legs are curled and that is what a scan can measure cleanly. From now it is taken head to heel. Ten centimetres of apparent growth is a change of ruler, and the NHS says so on this page. It is a good example of why a number without its definition is close to useless. Vernix now covers the skin - a white, greasy layer that protects it in fluid and eases the passage out. Kicking, punching, turning and thumb-sucking are all described this week. Administratively, week 20 is busy: the anomaly scan often lands here, the MAT B1 maternity certificate becomes available from a doctor or midwife and is what maternity pay is claimed with, and the whooping cough vaccination is typically offered around now, with the window running from 16 to 32 weeks. Halfway, on the 40-week convention. Worth marking.
Put your own dates on week 20
- Week 1 began
- Thu, Jan 1, 2026
- Week 20 begins
- Thu, May 14, 2026
- Week 20 ends
- Wed, May 20, 2026
- Estimated due date
- Thu, Oct 8, 2026
Dates run on ACOG's 280-day convention, the same arithmetic as the due date calculator. Share this week with your dates: /pregnancy/week-20?s=d:2026-10-08
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What the NHS describes this week
- The NHS puts the foetus at around 25.6 cm this week, and states that measurements are now taken from head to heel rather than head to bottom.
- Vernix, a white greasy layer, now covers the skin.
- Kicking, punching, turning and thumb-sucking are all described at this stage.
What happens around now
The anomaly scan is commonly offered around now. The MAT B1 maternity certificate is available from a doctor or midwife and is what maternity pay is claimed with. The whooping cough vaccination is usually offered around week 20, within a window from week 16 to week 32.
- Anomaly scan - offered between 18 and 21 weeks (NHS)
- Whooping cough vaccination - usually offered around week 20 (NHS)
- MAT B1 maternity certificate - available from a doctor or midwife (NHS)
On the measurement. The NHS publishes a length for each week from 4 to 40. Up to week 19 it is measured head to bottom (crown to rump, the measurement a scan takes while the legs are curled). From week 20 it is measured head to heel, which is why the figure jumps from 15.3 cm at week 19 to 25.6 cm at week 20 - the baby did not grow 10 cm in a week, the ruler changed. The NHS states the change on its week 20 page.
Where week 20 comes from
- NHS: week 20 of pregnancy
- NHS: 20-week screening scan
- NHS, Week-by-week guide to pregnancy — the guide these weeks are built from
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