Pregnancy trimester calculator
Trimesters are coarse buckets on top of gestational age. We use a common teaching split: 1st through 13w6d, 2nd 14w0d–27w6d, 3rd from 28w0d — disclosed because hospitals sometimes cut differently. Estimate only — not medical advice.
Enter a date to see an estimate.
What this page does
Enter LMP, EDD, or IVF transfer details; we compute GA and map it to first / second / third trimester with the weeks+days underneath.
Useful for leave conversations and “we’re announcing second trimester” texts — not for deciding which prenatal screen you need.
When to use
Work leave; announcing trimester; coarse timeline checks.
When not to
Screening medical necessity — clinician territory.
Assumptions
T1: GA day 0–97 (through 13w6d). T2: 14w0d–27w6d. T3: 28w0d onward. Conventions vary by source — we disclose ours.
Examples
LMP Jan 1, 2026 · as-of Jul 17, 2026 → Third trimester (~28w+).
EDD Dec 1, 2026 · as-of Jul 17, 2026 → Second trimester example.
Gotchas
Clinic dating after ultrasound can differ from browser math — the clinic wins for medical decisions.
Wrong input dates (retrieval vs transfer, spotting vs true LMP) slide every result.
Estimate only — not medical advice.
How this is calculated
The math stays in your browser. Enter the dates you actually know, read the estimate, and treat clinic ultrasound dating as the authority when it disagrees.
Nothing here is medical advice, a diagnosis, or a substitute for prenatal care.
- Enter the date(s) or numbers this page asks for.
- Check the assumptions under the result and on Methodology.
- Compare with your clinic portal if you have one.
- Use related tools if you only have a different kind of date.
- Confirm medical decisions with your clinician.
See Methodology for the dating table used across PregDateBase.
Related calculators
Fine-grained weeks: gestational age. Conversational week: pregnancy week calculator.
See also: gestational age calculator, pregnancy week calculator, and pregnancy calculator by weeks.
FAQ
- When does the second trimester start?
- Commonly 14 weeks 0 days in many patient materials.
- When does the third trimester start?
- Commonly 28 weeks.
- Do IVF pregnancies use different trimesters?
- Same buckets once GA is established.
- Why does another site disagree?
- Cutoff conventions differ by about a week sometimes.
- Fourth trimester?
- Postpartum slang — not a dating mode here.
- Is this medical advice?
- Estimate only — not medical advice. Confirm dating, testing, and medical decisions with your clinic.