How the dates are worked out
Pregnancy is counted from the first day of your last period, even though conception happens about two weeks later. A due date is 40 weeks (280 days) from that day.
From your last period
Due date = first day of last period + 280 days
If your usual cycle is longer or shorter than 28 days, we move the date by the difference. For a 32-day cycle, the due date is 4 days later. This is a rough adjustment. It assumes you ovulate about 14 days before your next period, which isn't true for everyone.
From IVF
Due date = transfer date + 266 days − embryo age
For a day-5 embryo, that's 261 days after transfer. Your fertility clinic will confirm your date.
From an ultrasound scan
Due date = scan date + 280 days − how far along the scan measured
A scan in the first trimester (up to about 14 weeks) is the most accurate way to set a due date. Scans later in pregnancy are less accurate, because babies grow at different rates.
If you also enter your period date, we compare the two. A scan up to 13 weeks and 6 days sets your due date, following the New Zealand Obstetric Ultrasound Guidelines. In the UK and Ireland, your due date is also normally set from your dating scan.
For a scan from 14 weeks onwards in New Zealand, the scan date is used when it differs from your period date by more than:
| Scan taken at | Scan preferred if the dates differ by more than |
|---|---|
| 14+0 to 15+6 weeks | 7 days |
| 16+0 to 21+6 weeks | 10 days |
| 22+0 to 27+6 weeks | 14 days |
| 28 weeks or later | 21 days |
Sources: New Zealand Obstetric Ultrasound Guidelines (Health NZ); ACOG/AIUM/SMFM Committee Opinion 700, Methods for estimating the due date. If you conceived by IVF, your due date comes from your transfer date and isn't changed by a scan.
From your care team
If you enter a due date from your midwife, doctor or scan, we use it as it is. A dating scan in early pregnancy is usually more accurate than a period date, which is why your team's date always comes first.
What the timeline shows
The timeline shows the usual windows for tests and checks, based on national guidance in New Zealand, the UK and Ireland. They're planning windows, not appointments. Your midwife or doctor arranges your care and may recommend a different schedule.
Your privacy
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Who checked this
The clinical content on this site was reviewed by a senior consultant obstetrician and gynaecologist practising in New Zealand. Last reviewed [date].