How the estimate works

The formulas

• A pregnancy is counted from the first day of the last period. This is week 0. A full pregnancy is 280 days, or 40 weeks.
• Last period method: due date = first day of the last period + 280 days + (your cycle length - 28 days).
• Conception method: due date = conception date + 266 days.
• IVF method: due date = transfer date + 261 days for a day 5 embryo, or + 263 days for a day 3 embryo.
• How far along you are = today - (due date - 280 days). The tool shows this in weeks and days.
• First trimester: 0 weeks 0 days to 13 weeks 6 days. Second trimester: 14 weeks 0 days to 27 weeks 6 days. Third trimester: 28 weeks 0 days and later.
• Full term: 39 weeks 0 days to 40 weeks 6 days.
• Ovulation day = next period - 14 days. The fertile window is the 5 days before ovulation and the day of ovulation.
• If a pregnancy starts in a cycle, the due date is the ovulation day + 266 days.

The limits

• Cycles are not the same for every person, and they change from month to month.
• The day of ovulation changes. The tool cannot know your own day.
• A scan from your doctor gives a more exact date than a calculator. If the two dates are different, follow your doctor.
• The tool gives an estimate only. It is not medical advice and it is not birth control.

Sources

• ACOG Committee Opinion No. 700: Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists, 2017. The 280 day count and the use of a scan.
• NHS: Pregnancy due date calculatorThe due date, week by week.
• NHS: Trying for a babyOvulation and the days when pregnancy is likely.
• Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-1521The fertile window of 6 days that ends on the ovulation day.

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