Pregnancy Trimester Calculator — Which Trimester Am I In?

Gestational age is counted from your last period, not from conception

This is the detail that confuses nearly everyone. Standard obstetric dating counts from the first day of your last menstrual period (LMP), not from the moment of conception. Since ovulation typically occurs around two weeks into a cycle, gestational age runs roughly two weeks ahead of the actual age of the embryo.

The consequence catches people out: at the moment a pregnancy test turns positive you are typically already counted as four to five weeks pregnant, and the first two of those weeks preceded conception entirely. A pregnancy described as 40 weeks involves about 38 weeks since conception.

The convention exists for a practical reason — the LMP date is something you can usually know, whereas the exact date of conception normally is not. You can see the offset directly in the arithmetic this tool uses: a due date is LMP plus 280 days, or conception plus 266 days, and the difference between those two figures is exactly the fourteen days described above.

Trimester boundaries are not universally agreed

This tool divides pregnancy into weeks 1–12, weeks 13–27, and weeks 28–40, and marks anything past 40 weeks as overdue. Other clinical sources and other countries place the boundaries a week or so differently, so another tool may put you in a different trimester when you are close to a transition. Neither is wrong — it reflects genuine variation in medical practice rather than an error, and nothing about your pregnancy changes at a boundary.

The divisions are useful because they loosely track distinct phases: organ formation early on, a middle stretch that is often the most comfortable, and a final phase of rapid growth. The tool shows a short set of highlights for each phase — typical scan timings and the sorts of changes common in that stretch — as general orientation, not as a checklist to measure yourself against.

A due date is an estimate, not a deadline

The due date here is LMP plus 280 days, which is Naegele's rule and the standard most clinicians start from. But only a small minority of babies arrive on the estimated date — commonly cited as around one in twenty — and a normal term birth spans roughly 37 to 42 weeks. The date is the centre of a wide range, not a target.

Two things make an LMP-based estimate less reliable: irregular cycles, and a cycle whose length differs much from 28 days, since Naegele's rule quietly assumes ovulation around day 14. If you have had a dating ultrasound, particularly in the first or early second trimester, that measurement is more accurate than any calculation from dates — your clinician's date supersedes this or any other calculator.

Information for planning, not a substitute for prenatal care

This tool does date arithmetic. It tells you where you are on the timeline and roughly what phase that corresponds to. It is not medical advice, and it does not replace prenatal care, regular check-ups with your midwife or doctor, or any screening or monitoring your clinician recommends.

Anything clinical — symptoms, questions about how the pregnancy is progressing, or concerns about your health — belongs with your healthcare provider, who knows your history and can actually examine you. A calculator knows only two dates.

Trimester boundaries are counted from the same starting point as the due date, so the two should always agree — if they do not, the start date is wrong in one of them. Check it with the due date calculator.