# 3 Steps to Calculate Your Gestational Age

*Published 2026-08-10.*

> Learn how to accurately track your pregnancy timeline using your last menstrual period or ultrasound results. This guide breaks down the math behind your estimated due date, current trimester, and upcoming pregnancy milestones.

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Doctors track pregnancy using gestational age, a standardized measurement that gives everyone a common baseline. It differs from fetal age, which actually starts at the moment of conception. Instead, gestational age begins counting on the first day of your last menstrual period, often referred to as your LMP. Because ovulation usually happens around 14 days after a menstrual period begins, the first two weeks of your official gestational age pass before you are even pregnant. A standard full-term pregnancy lasts exactly 40 gestational weeks.

If you want to know exactly how far along you are today, running your numbers through a calculator makes it easy to find your current week, trimester, and estimated due date. The tool handles the two most common dating methods used in obstetrics: your last menstrual period or a medical ultrasound. 

## Step 1: Choose your input method

The first thing you need to do is tell the calculator how you are tracking the pregnancy. You have two choices: your last period or an ultrasound scan.

The LMP method is the standard way to calculate a due date early on, usually before you have your first dating scan. It assumes a typical 28-day menstrual cycle where ovulation happens exactly on day 14. Since actual cycle lengths vary widely from person to person, this method usually has a margin of error of a few days.

If your obstetrician or midwife has already given you a gestational age based on a dating scan, use the ultrasound method. The American College of Obstetricians and Gynecologists notes that a first-trimester ultrasound — one performed before week 14 — is typically the most accurate way to estimate a due date. Medical imaging measures the actual size of the fetus rather than relying on cycle averages. If you have those numbers from your doctor, use them.

## Step 2: Enter your dates

The information you type in depends entirely on the method you picked.

For the LMP method, simply enter the date your last period began. Check the year twice, especially if your last period started late in the previous calendar year. A simple typo in the year will throw the whole calculation off.

If you select the ultrasound method, grab the paperwork or notes from your medical appointment. You will need to fill out three specific fields to get an accurate result:

1. Scan date: The exact day your ultrasound took place.
2. Gestational weeks: The number of full weeks you were measuring on the day of the scan.
3. Gestational days: Any extra days beyond the full weeks.

For instance, if your doctor mentioned you were measuring 8 weeks and 3 days pregnant at your appointment, type 8 into the weeks field and 3 into the days box.

## Step 3: Review your results

After you hit calculate, the tool runs standard medical formulas to figure out exactly where you are in the pregnancy right now.

Your current gestational age appears first, displayed in weeks and days. If you went with the LMP method, the tool counts the exact number of days between your last period and today, then divides by 7. If you used the ultrasound option, the math is slightly different. The calculator works backward from your scan date and measurements to pinpoint an equivalent LMP date, then projects forward to today.

Your estimated due date is the day you hit exactly 40 weeks. To find this, the calculator uses Naegele's rule, a standard obstetrics formula that adds exactly 280 days to your actual or equivalent LMP date. Keep in mind that this is just a statistical estimate. Only about 5 percent of babies arrive on their exact due date. The vast majority are born somewhere in the window of two weeks before to two weeks after that specific day.

You will also see a countdown showing the days remaining until you reach the 40-week mark.

Below that, the tool lists your current trimester. The first trimester covers weeks 1 through 12, the second spans weeks 13 through 27, and the third runs from week 28 up to week 40.

Finally, you get a look at upcoming milestones. The calculator points out the next major medical or developmental event based on your current week.

## A complete worked example

Let's walk through a real scenario using the LMP method to see the math in action.

Assume today is April 19, 2026. You select the LMP method and enter your last menstrual period as October 1, 2025. 

First, the calculator counts the days between October 1, 2025, and April 19, 2026. That is exactly 199 days. Next, it converts those days into weeks by dividing by 7 (199 ÷ 7). The result is 28 weeks with a remainder of 3 days. That makes your current gestational age 28 weeks and 3 days.

Since week 28 is the start of the final stage of pregnancy, the tool shows you are in the third trimester.

To find your due date, it applies Naegele's rule, adding 280 days to October 1, 2025. This lands on an estimated due date of July 8, 2026. Finally, it subtracts your 199 elapsed days from the 280 total days to show you have 81 days left until delivery.

## Tracking your pregnancy milestones

As your gestational age increases, you pass several standard medical benchmarks. The calculator follows a fixed schedule to let you know what is coming next.

| Week | Milestone |
|---|---|
| 6 | Heartbeat detectable |
| 10 | Embryo period ends — now a fetus |
| 12 | End of first trimester |
| 20 | Anatomy scan (mid-pregnancy ultrasound) |
| 24 | Viability threshold |
| 28 | Third trimester begins |
| 37 | Full term |
| 40 | Estimated due date |

If you are at 18 weeks, your next listed milestone is the anatomy scan at week 20. Pass week 20, and the tool updates to show the viability threshold at week 24.

While this helps you plan and track your timeline at home, it is for informational purposes. Always talk to your healthcare provider for clinical dating and prenatal care.

Plan your timeline and track your upcoming milestones with the [Gestational Age Calculator](/health/gestational-age/).

## Frequently asked questions

### What is the difference between gestational age and fetal age?

Gestational age begins on the first day of your last menstrual period, which is usually about two weeks before you actually conceive. Fetal age starts at the exact moment of conception. Doctors use gestational age as a standard baseline because it is easier to track.

### Which method is more accurate for calculating a due date?

A first-trimester ultrasound is generally considered the most accurate way to estimate your due date. This method measures the actual size of the fetus rather than relying on menstrual cycle averages. If you have dating scan results from your doctor, you should use those instead of your last period.

### Will my baby be born exactly on my estimated due date?

It is highly unlikely that you will deliver exactly on your estimated due date. Only about five percent of babies arrive on that specific day. Most babies are born within a window of two weeks before or two weeks after the estimated date.


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