Every method reduces to one date
Whatever you start from, the calculator finds day zero, the LMP-equivalent, and counts days from there. From a last period that date is the period itself. From a due date it is 280 days earlier. From a scan it is the scan date minus the age measured. From an IVF transfer it is the transfer date minus 14 days minus the embryo's age. Once day zero is fixed, gestational age on any date is simple subtraction.
Which method to trust
ACOG and NICE both recommend an early ultrasound as the most accurate dating method. If your scan date and your period date disagree by more than about five to seven days in the first trimester, clinics keep the scan. IVF dating is exact because the embryo's age is known. Last-period dating is the least precise because it assumes ovulation on day 14; if you know your ovulation or conception date, the due date by conception calculator will be closer.
Frequently asked questions
What is gestational age?
Gestational age is how far along a pregnancy is, counted from the first day of the last menstrual period rather than from conception. It is written as weeks plus days, for example 12+3. A full-term pregnancy is 37 to 42 weeks of gestational age, with the due date set at exactly 40+0.
How is gestational age calculated from a due date?
The due date is 40 weeks (280 days) of gestational age by definition, so the calculator subtracts 280 days from the due date to find day zero, then counts forward to the date you choose. Gestational age from a due date is exact whenever the due date itself is trusted.
How is gestational age calculated from an ultrasound?
A dating scan measures the embryo or fetus and reports a gestational age on the day of the scan, for example 8 weeks 3 days. The calculator subtracts that age from the scan date to find day zero, then counts forward. First-trimester scans are accurate to within about five days.
How is gestational age counted after IVF?
An embryo transferred on day 5 is already 2 weeks 5 days of gestational age on the transfer day, because gestational age includes the two weeks before ovulation plus the embryo's age. A day-3 embryo is 2 weeks 3 days at transfer. The calculator handles both.
What is the difference between gestational age and fetal age?
Fetal (or embryonic) age counts from conception and is about two weeks less than gestational age. Clinics, scans, and pregnancy apps almost always use gestational age. If a source says you are 10 weeks pregnant, that is gestational age; the embryo is about 8 weeks old.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.