The formula
How to calculate gestational age
Gestational age is measured from the first day of your last menstrual period, not from conception. That convention adds roughly two weeks to the actual age of the pregnancy, and it is why a "40-week" pregnancy involves about 38 weeks of fetal development.
Midwives write it as weeks plus days — 22+3 means 22 completed weeks and 3 days. The full term window runs from 37+0 to 41+6, with 40+0 as the nominal due date.
What to enter:
- Days since the first day of your last period (days)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
If an exact figure for one of the fields is not available, a careful estimate is a reasonable starting point, since the calculator recomputes instantly the moment a more accurate number is available — nothing here depends on getting every field exact on the first pass.
Why gestational age matters
The formula behind gestational age comes from published research rather than being invented for this page, and it is presented here exactly as it appears in the literature it is drawn from — the value of a calculator is applying it correctly and instantly, not reinventing it.
This tends to get checked before or after a change in routine — a new training block, a change in diet, a different stage of a pregnancy — as a way of putting a figure on something that would otherwise only be felt rather than measured.
None of the figures produced on this page are intended to replace a proper assessment, and nothing here should be read as a diagnosis or a treatment recommendation. What a calculator like this is good for is preparation — arriving at an appointment with an actual number already in hand, rather than a vague sense of a change, tends to make that conversation more useful for everyone involved.
Where a result depends on more than one measurement, small errors in each input tend to compound rather than cancel out — a slightly mismeasured waist and a slightly mismeasured height, taken together, can shift a derived figure more than either error would on its own. Taking each measurement carefully, and ideally twice, is worth the extra minute it takes.
Worked example
Work through the defaults on this page:
- Days since the first day of your last period: 158 days
That gives:
- Completed weeks: 22 weeks
- Plus days: 4 days
- Days to the 40-week due date: 122 days
Those starting figures exist only to show the calculation working, not as a target or a reference value to compare yourself against — replace them with your own measurements in the calculator above and the same formula applies exactly as shown.
Reading the result
Only about 4% of babies arrive on the due date itself. Roughly 80% arrive within the two weeks either side of it, which is why the date is better treated as the middle of a range.
Where this goes wrong. LMP dating assumes a 28-day cycle with ovulation on day 14. A dating scan in the first trimester is more accurate and is what your maternity notes will use if the two disagree. This calculator is general information, not clinical advice.
As with any general-purpose formula, the result here is an estimate derived from population averages, not a measurement specific to you — a reasonable starting point for a conversation with a qualified professional, not a replacement for one. Nothing on this page is medical advice.
Because the date of the last period is usually known and the date of conception is not. The convention predates ultrasound and has been kept for consistency across all pregnancy guidance.
A first-trimester scan measuring crown-rump length is accurate to within about five days. After around 14 weeks the accuracy falls off, since babies begin to vary more in size.
The headline figure is completed weeks. With 158 days days since the first day of your last period, that comes to 22 weeks. Change any field and the figure moves with it.
It uses a published formula applied exactly as defined, so the arithmetic itself is precise — but the formula is a population-level estimate, and individual results can vary from it for reasons the formula has no way to account for. Treat the output as a reasonable estimate rather than a measured or diagnostic value.
For most people, yes — a single reading is a snapshot, while the same measurement repeated consistently over weeks or months shows a trend, which is generally more informative than any one figure taken in isolation.
Small differences between calculators usually come down to which published formula or reference dataset each one uses — several equations in this area exist in more than one accepted version. This page states the specific formula used in the section above, so it is possible to check directly whether another source is using the same one.