NHS pregnancy dating and scan pathway
Under the UK National Health Service (NHS), pregnancy is calculated from the first day of your last period until your 12-week dating scan (11+2 to 14+1 weeks). At this scan, the sonographer measures the baby’s crown-rump length (CRL) to give you your official NHS delivery date.
Key NHS antenatal milestones include the initial midwife booking appointment (completed by 10 to 12 weeks), the dating ultrasound scan, and the 20-week screening scan (anomaly scan between 18+0 and 20+6 weeks).
UK Statutory Maternity Leave can start at any point from 11 weeks before your expected week of childbirth (EWC), provided you notify your employer by the 15th week before your due date.
Worked example: UK maternity planning
With an estimated due date of 15 June, the expected week of childbirth (EWC) starts on the Sunday prior to that date.
The earliest statutory date to begin maternity leave (11 weeks prior) falls in late March, while the NHS 20-week anomaly scan takes place in late January.
UK pregnancy inputs
Calculation Method
Select LMP or ultrasound scan date.
Cycle Length
Your typical cycle duration in days.
CALCULATOR QUESTIONS
NHS pregnancy due date & scan calculator questions
Can my NHS due date change after my first scan?
Yes. If the ultrasound measurement of the crown-rump length (CRL) differs by more than 7 days from your LMP calculation, the NHS will officially adjust your due date to match the scan.
When do I receive my MAT B1 certificate?
Your midwife or GP issues your MAT B1 certificate after 20 weeks of pregnancy, usually around the time of your 20-week scan, confirming your expected delivery date for maternity pay.