How are weeks of pregnancy counted?

Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. Conception typically occurs around week 2. So at 4 weeks pregnant, you may have actually conceived only about 2 weeks ago.

How many weeks pregnant am I if my last period started 10 weeks ago?

10 weeks. Pregnancy is counted from the first day of your last period, so the number of weeks since that day is how many weeks pregnant you are, even though conception happened about 2 weeks later.

What is gestational age vs foetal age?

Gestational age counts from the LMP and is used by healthcare providers. Foetal age (embryonic age) counts from conception and is typically about 2 weeks less than gestational age. This calculator uses gestational age, the standard medical measure.

Is my data sent to a server?

All calculations run locally in your browser. Our servers are not involved at any point.

Pregnancy week calculator

Find out exactly how many weeks and days pregnant you are right now. Enter your last menstrual period date or conception date and this calculator shows your gestational age in weeks and days, your current trimester, and your estimated due date. All calculations run locally in your browser. Confirm your dates with a healthcare provider.

Calculate from

First day of last period

Results are for informational purposes and do not constitute medical advice.

How it works

Gestational age is the number of whole calendar days from the first day of your last period to today, split into completed weeks and remaining days: 75 days shows as 10 weeks, 5 days. In conception mode the count starts 14 days before the conception date. Once the estimated due date is today or in the past, the result shows 40 weeks (term) instead of a running count. All date arithmetic runs in your browser.

Processing runs in your browser

All calculations happen locally in your browser tab. Our servers are not involved at any point.

This tool provides an estimate only. Confirm your due date and gestational age with a qualified healthcare provider. Ultrasound measurements may result in a different estimate.

Technical specification

Naegele's rule dates a pregnancy from the first day of the last menstrual period and places the estimated due date 280 days (40 weeks) later. The American College of Obstetricians and Gynecologists describes this dating method, and its preference for early ultrasound when the two disagree, in Committee Opinion No. 700, Methods for Estimating the Due Date (2017).

Formula: LMP + 280 days; conception + 266 days
Trimesters: 0–12, 13–26, 27+ completed weeks
Date handling: Whole calendar days, local time zone
Browser API: JavaScript Date. No library

Related operations

To find an exact age from a past birthday, try the age calculator. For working out gaps between any two dates, use the date calculator. To work out a pre-pregnancy BMI from your usual height and weight, use the BMI calculator.

Frequently asked questions

How are weeks of pregnancy counted?: Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. Conception typically occurs around week 2. So at 4 weeks pregnant, you may have actually conceived only about 2 weeks ago.
How many weeks pregnant am I if my last period started 10 weeks ago?: 10 weeks. Pregnancy is counted from the first day of your last period, so the number of weeks since that day is how many weeks pregnant you are, even though conception happened about 2 weeks later.
What is gestational age vs foetal age?: Gestational age counts from the LMP and is used by healthcare providers. Foetal age (embryonic age) counts from conception and is typically about 2 weeks less than gestational age. This calculator uses gestational age, the standard medical measure.
Is my data sent to a server?: All calculations run locally in your browser. Our servers are not involved at any point.

Last reviewed September 30, 2026

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