How are the trimesters defined?

The first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. These are measured in gestational weeks from your last menstrual period.

How is the due date calculated?

It uses Naegele's rule: 280 days (40 weeks) added to the first day of your last menstrual period. Ultrasound dating can differ, so always confirm with your clinician.

What is gestational age and how is it measured?

Gestational age is how far along the pregnancy is, counted from the first day of the last menstrual period — not from conception. It is shown here in completed weeks plus extra days, e.g. 12 weeks 3 days.

Why is pregnancy dated from the last period rather than conception?

The LMP is a date most people can identify, while conception usually happens around two weeks later and is harder to pin down. Dating from the LMP gives a consistent, standard reference used in clinical care.

How many weeks is each trimester?

Roughly: the first trimester is about 13 weeks, the second about 14 weeks (weeks 14–27), and the third about 13+ weeks (week 28 to birth around week 40), for a typical 40-week pregnancy.

Can the calculator tell me which trimester I am in now?

Yes. From your LMP it works out today's gestational age and labels you as being in the first, second or third trimester, or post-term if you are past the due-date window.

Is any information uploaded?

No. The calculation happens entirely in your browser and nothing is stored or sent anywhere.

Pregnancy Trimester Calculator

Pregnancy trimester calculator

Enter the first day of your last menstrual period (LMP) to see your estimated due date, your gestational age in weeks and days, and the exact dates each trimester begins and ends. It is a quick way to know how far along you are and what stage of pregnancy you are in today.

How it works

Dating follows the standard Naegele’s rule and fixed trimester boundaries, counted in gestational weeks from the LMP:

due date         = LMP + 280 days (40 weeks)
gestational age  = days since LMP, shown as weeks + days
first trimester  = weeks 0–13   (up to day 97)
second trimester = weeks 14–27  (days 98–195)
third trimester  = week 28 onward (day 196 to birth)

Example

For an LMP, with 98 days elapsed (14 weeks 0 days):

Gestational age: 14 weeks 0 days
Stage: Second trimester (just started)
Due date: LMP + 280 days

Gestational age	Trimester
0–13 weeks	First
14–27 weeks	Second
28 weeks to birth	Third

Ultrasound dating may shift these dates, so use this as a guide and confirm details with your healthcare provider. Everything is computed privately in your browser.