Pregnancy calculator
Gestational age is measured from the first day of the last period, roughly two weeks before conception actually happens. It survives because the period is a date most people can name and conception is not. It also means a pregnancy described as "four weeks" involves an embryo about two weeks old — which is why the dating scan, usually between 8 and 14 weeks, so often shifts the due date by a few days.
An estimate from a standard formula. Only a dating scan can date a pregnancy properly, and about one birth in twenty-five happens on the estimated date. Always follow the advice of your midwife or doctor.
A pregnancy due date is the first day of the last menstrual period plus 280 days, adjusted if your cycle is not 28 days. Gestational age is counted from that same date, which is about two weeks before conception — so a pregnancy at four weeks contains an embryo roughly two weeks old.
How to work out how far along you are
Naegele's rule has been in use since the early nineteenth century and remains the standard, but it carries assumptions worth knowing. It assumes ovulation on day 14, which is only true for a 28-day cycle, and it assumes the cycle is regular. For a 35-day cycle ovulation is nearer day 21, and the due date moves a week later — which is what the cycle field here adjusts for. A first-trimester dating scan measures the embryo directly and is more accurate than any date arithmetic; where the two disagree by more than about five days, the scan is normally used.
Questions
First day of the last period plus 280 days, or 40 weeks. A cycle longer or shorter than 28 days shifts it by the difference.