Formulas and sources
- Due date: Naegele’s rule. First day of the last period plus 280 days, adjusted for cycle length. Source: ACOG Committee Opinion 700.
- Conception date: due date minus 266 days.
- IVF due date: transfer date plus 266 days, minus the age of the embryo in days. Source: ACOG Committee Opinion 700.
- Ovulation and fertile window: ovulation is estimated at 14 days before the next period; the fertile window is the 5 days before ovulation and ovulation day. Source: NHS.
- Baby growth percentiles: WHO Child Growth Standards, using the published LMS values for birth to 24 months.
- Week by week and month by month guides: ACOG, NHS, WHO and CDC guidance.
Every calculator page explains its own formula and lists its sources at the end.
What we check before publishing
- That each formula matches the published guidance it cites
- That results match worked examples calculated by hand
- That advice on when to see a doctor comes from the cited source
- That sources are up to date and reliable
How often pages are updated
Each page lists its sources. We re-check every page at least once a year, and sooner when major guidance from organisations such as ACOG, WHO or the NHS changes.
Who writes this site
DueNest is an independent website. Our content is written from published medical guidance and is not reviewed by a doctor, so it cannot replace advice from your own doctor or midwife. If you are a doctor, midwife or nurse and would like to review our content, please get in touch through our contact page.