How Many Weeks Pregnant Am I?

The counting convention

Gestational age is measured from the first day of your last menstrual period (LMP) — not from conception, and not from the positive test.

The first six weeks

Week 0–2 · period, then ovulation Week 2–4 · conception, implantation Week 4–6 · missed period, positive test

Two consequences follow. On the day you conceive you are already described as 2 weeks pregnant. And a "40-week" pregnancy contains roughly 38 weeks of embryonic and fetal development.

Working out your week

days since LMP ÷ 7 = weeks + days
174 ÷ 7 = 24 remainder 6 → 24w6d

Clinicians always write it as weeks and days, because in obstetrics a few days genuinely matter. "24 weeks" and "24w6d" are a week apart in practice.

Get your exact week, day and trimester.Pregnancy calculator →

The trimesters

Trimester	Weeks	Roughly
First	0w0d – 13w6d	Organ formation; most symptoms peak
Second	14w0d – 27w6d	Anatomy scan; movement felt
Third	28w0d – birth	Growth, positioning, more frequent visits

Why charts disagree: trimester boundaries are conventions rather than biological events, and different organisations place them a week apart. If your clinic's chart and an app differ by a few days, neither is wrong — they are using different cut-offs.

What "term" means

Since 2013 the medical definition of a term pregnancy has been split into four categories, because outcomes differ measurably between them:

Category	Weeks
Early term	37w0d – 38w6d
Full term	39w0d – 40w6d
Late term	41w0d – 41w6d
Post-term	42w0d and beyond

When dating gets adjusted

LMP dating assumes a 28-day cycle with ovulation on day 14. Plenty of cycles do not work that way, so dating is revised when:

Cycles are long, short or irregular. Ovulating on day 21 rather than day 14 makes the LMP-based date a week late.
The LMP is uncertain. Common after hormonal contraception or while breastfeeding.
An early scan disagrees. A first-trimester ultrasound is accurate to about ±5–7 days, and takes precedence over LMP when the gap exceeds that.
Conception is known precisely. With IVF, the transfer date gives an exact age, so LMP is not used at all.

This article explains how the dates are calculated — it is general information, not medical advice. Your midwife or doctor's dating is the one that counts, and any concern about your pregnancy should go to them rather than a calculator.

Related: how the due date itself is derived from the same LMP figure, and how to work backwards to the likely conception date.

Run your own numbers

Pregnancy Due Date Conception

FAQ

How do I know how many weeks pregnant I am?

Count the days from the first day of your last menstrual period to today, then divide by 7. The whole number is your week and the remainder is the days — 174 days is 24 weeks and 6 days, written 24w6d.

Why is pregnancy dated from my last period instead of conception?

Because the date of the last period is something most people can identify, while the exact day of conception usually is not. The convention has been standard in obstetrics for two centuries, and it is why a 40-week pregnancy contains only about 38 weeks of actual fetal development.

How many weeks pregnant am I when I get a positive test?

Typically 4 to 5 weeks. Most tests turn positive around the time of a missed period, which is roughly two weeks after ovulation and therefore about four weeks after your last period began.

When does each trimester start and end?

The first trimester runs from 0w0d to 13w6d, the second from 14w0d to 27w6d, and the third from 28w0d until birth. Different sources shift these boundaries by a week or so, so a clinic's chart may not exactly match an app's.

Can my due date change after an ultrasound?

Yes. A first-trimester scan measures the embryo directly and is accurate to within about five to seven days. If it disagrees with your period-based date by more than that, clinicians usually redate the pregnancy to the scan, which is considered more reliable.

Sources

Primary references used for the figures and rules on this page.

Methods for Estimating the Due Date — ACOG
Pregnancy — NIH — NICHD