How our calculators work, and our sources

Pregnancy due date

From the last period: the first day of the last period plus 280 days (40 weeks), moved earlier or later by how far your cycle is from 28 days (Naegele's rule). From the conception date: 266 days after it. For IVF: the transfer date plus 261 days for a day 5 embryo, or 263 days for a day 3 embryo.

Trimesters are counted as the first until week 14, the second from week 14 and the third from week 28. Sources differ slightly on these boundaries.

Check-up schedule and vaccine window

The check-up schedule follows the World Health Organization's 2016 recommendation of at least eight contacts: by week 12, then at weeks 20, 26, 30, 34, 36, 38 and 40. Countries and doctors schedule care differently, so follow your doctor.

The Tdap vaccine is advised in every pregnancy, preferably between weeks 27 and 36.

Ovulation and fertile window

Ovulation is estimated about 14 days before the next period: the first day of the last period plus the cycle length minus 14 days. The fertile window is the five days before ovulation, ovulation day, and one day after as a margin.

The highest-chance days (the two days before ovulation and ovulation day) and the window itself come from Wilcox and colleagues, New England Journal of Medicine, 1995, a study of 221 women, in which pregnancy only occurred after intercourse in the six days ending on the day of ovulation. This is an estimate that assumes a regular cycle.

Baby age and milestones

Age is counted in whole calendar months and the days left over. For a baby born before 37 weeks, the corrected age is counted from the original 40 week due date, that is the date of birth moved forward by the weeks born early.

The milestone ranges (sitting without support, standing with assistance, hands-and-knees crawling, standing alone and walking alone) are the windows of achievement from the WHO Multicentre Growth Reference Study, 2006, which followed 816 healthy children from Ghana, India, Norway, Oman and the USA. They describe normal variation, not a pass mark.

The breastfeeding lines follow the WHO recommendation of exclusive breastfeeding for the first six months, then complementary foods with continued breastfeeding up to two years or beyond.

Hijri dates

Hijri ages use the Umm al-Qura calendar, the official calendar in Saudi Arabia. It can differ by a day or two from other calendars or from local moon sighting.

Corrections

If you spot a mistake in a figure or an explanation, we want to fix it. Accuracy matters more to us than anything else on this site.