Tools · Pregnancy

Your Due Date, Current Week & Trimester — Free Calculator

Enter the first day of your last period to see your estimated due date, current pregnancy week, and trimester — free, based on Naegele's rule.

First day of your last menstrual period Average cycle length (days)

Enter the first day of your last period to see your estimated due date.

This is an estimate based on Naegele's rule. An early ultrasound gives the most accurate dating — always confirm with your healthcare provider.

How the Due Date Is Calculated

This calculator uses Naegele's rule, the standard method used in obstetrics: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle differs from 28 days, the calculator adjusts the estimate by the difference. Pregnancy is measured in gestational age — counted from your LMP, not from conception — which is why you are considered "2 weeks pregnant" around the time of conception.

Remember that a due date is a single point in a normal range. A full-term pregnancy is anywhere from 37 to 42 weeks, and only about 5% of babies arrive on the exact estimated date. Use the result to plan, but stay flexible.

Frequently Asked Questions

30 weeks is how many months?

About 6.9, so you are in your seventh month. Dividing by four gives 7.5, and that is the same arithmetic that turns 40 weeks into ten months.

How is my due date calculated?

The most common method is Naegele's rule: your estimated due date is 280 days (40 weeks) from the first day of your last menstrual period (LMP), assuming a regular 28-day cycle. This calculator uses that method. Only about 1 in 20 babies arrive exactly on their due date — it is an estimate, not a deadline.

What if my cycle is not 28 days?

If your cycle is consistently longer or shorter than 28 days, your due date may shift by the difference. For example, a 32-day cycle could push the date about 4 days later. An early ultrasound (especially in the first trimester) gives the most accurate dating and is what your care provider will rely on.

How accurate is a due date calculator?

An LMP-based calculator gives a solid estimate for people with regular cycles who remember their last period. However, ultrasound dating in early pregnancy is more accurate, particularly for irregular cycles. Treat the calculated date as an approximate guide and confirm with your healthcare provider.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last period and is what pregnancy weeks refer to (e.g. "20 weeks pregnant"). Fetal age (or conceptional age) is counted from conception, which is roughly 2 weeks later. Medical care and this calculator use gestational age.

Source: ACOG — Methods for Estimating the Due Date

Why 40 weeks is not ten months

A calendar month averages 4.35 weeks, not 4. Dividing by four is where the extra month comes from: 40 ÷ 4 = 10, but 40 ÷ 4.35 = 9.2. Forty weeks is a little over nine months — which is part of why the last month feels longer than it should.