CalcVitalsTrimester Calculator — Dates for All Three Trimesters
Pregnancy's 40 weeks divide into three trimesters: the first runs to the end of week 13, the second covers weeks 14–27, and the third runs from week 28 to the birth. Enter your last period and this page dates each stage for you and marks where you stand today.
How it works
The trimester division is bookkeeping, not biology — development is continuous — but it is bookkeeping that organises almost everything: which screening tests happen when, when viability milestones pass, how symptoms are interpreted, and how the flood of pregnancy information is sorted. Knowing your dates in this frame makes every appointment letter and week-by-week guide immediately legible.
Each trimester has its reputation. The first carries the highest miscarriage risk (most losses happen here, overwhelmingly from chromosomal causes no behaviour controls) along with nausea and exhaustion, and it is when the embryo's organs form — the reason folate matters most early. The second is usually the most comfortable: nausea fades, energy returns, the anatomy scan happens around week 20, and movement is first felt. The third is growth, weight gain and preparation — Braxton Hicks, breathlessness as the uterus crowds the diaphragm, and the slow countdown past full term at 39–40 weeks.
If your dates here disagree with your care team's, theirs govern: they may be working from an ultrasound-derived due date rather than your last period. Re-run this page from a period date that matches their due date (the due date calculator works the same arithmetic) and everything will align.
The method
Trimester 1: LMP through LMP + 97 days (13w6d). Trimester 2: day 98–195 (14w0d–27w6d). Trimester 3: day 196 (28w0d) to due date at day 280. Current-trimester label from gestational age today.
Frequently asked questions
When exactly does the second trimester start?
At 14 weeks 0 days by the convention used here (some sources say 13 weeks). It is a rounding convention on a continuous process — a few days either way changes nothing medically.
Which trimester is the safest for travel?
The second trimester is generally considered the most comfortable window for travel — nausea has typically passed and the late-pregnancy risks have not arrived. Airlines commonly restrict travel after 36 weeks; check your carrier and clinician.
What happens in each trimester at check-ups?
First: dating scan, initial bloods, screening offers (around 11–14 weeks). Second: the detailed anatomy scan near 20 weeks, glucose screening around 24–28 weeks. Third: growth checks, position, and birth planning, with visit frequency increasing toward term.