The formula
How to calculate pregnancy trimester
Pregnancy is conventionally divided into three trimesters of roughly thirteen weeks each. The boundaries are approximate and vary slightly between sources, but the first trimester always ends around week 13 and the third begins around week 28.
The divisions mark clinical rather than biological milestones. Miscarriage risk falls sharply at the end of the first trimester; viability outside the womb begins around the start of the third.
What to enter:
- Weeks pregnant (weeks)
No submit button: type and the answer moves. Your inputs end up in the link, so the page can be shared already filled in.
Consistency in how the underlying measurements are taken matters more than precision to the last decimal — measuring at the same time of day, in the same state of rest or hydration, makes readings taken weeks apart genuinely comparable in a way that a single very precise reading on its own is not.
Why pregnancy trimester matters
The formula behind pregnancy trimester 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.
It is most useful tracked over time rather than read once — a single reading tells you where you are today, while the same calculation repeated weeks or months apart, using the same method each time, shows the direction things are actually moving in.
A single number like this is most useful placed in context rather than read in isolation. Reference ranges exist because bodies vary considerably and still count as healthy across a wide spread of values, so a result sitting outside a commonly quoted range is a prompt to look more closely rather than a verdict on its own. The same figure, tracked consistently over time, tends to say more than any single reading ever will.
In practice, this kind of figure is often calculated more than once with slightly different inputs to see how sensitive it is — trying a measurement taken this morning against one taken yesterday evening, for instance, to see how much normal daily variation actually moves the result. That habit is a good one: it builds a feel for how much a single reading can be trusted on its own before drawing any conclusion from it.
Worked example
Take the figures the calculator starts with:
- Weeks pregnant: 22 weeks
That gives:
- Trimester: 2
- Weeks to the due date: 18 weeks
- Pregnancy completed: 55 %
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
Week 24 is the point at which UK neonatal units will generally provide active resuscitation, and it is also the legal limit for most abortions. Week 37 marks full term, after which delivery is no longer considered premature.
Where this goes wrong. Different sources put the second trimester start at week 13 or week 14, so a week either way is normal disagreement rather than an error. Your maternity notes are the authority for your own pregnancy.
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.
Most UK sources say week 14, some say week 13. The distinction matters little clinically — the meaningful change is the end of the high-risk early period.
Thirty-seven to forty-two weeks. Within that, 39–40 weeks is now described as "full term", with 37–38 as "early term" following a 2013 change in definitions.
The answer it gives you is trimester. With 22 weeks weeks pregnant, that comes to 2. 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.