Introduction
A gestational age calculator helps you find out how far along a pregnancy is. Gestational age is the number of weeks and days since the first day of the last menstrual period (LMP). Doctors use this number to track the baby's growth, plan important tests, and estimate the due date.
This calculator lets you start from whichever value you already know. You can enter your LMP date, your current gestational age, a due date, an ultrasound date, or an estimated conception date. It will then figure out everything else for you, including your trimester, estimated due date, days remaining, and key milestone dates throughout your pregnancy.
You can also compare two dating methods side by side. For example, you can check your LMP-based date against an ultrasound-based date to see if they match. If there is a big difference, the tool will flag it so you can talk to your doctor.
How to Use Our Gestational Age Calculator
Enter one known pregnancy date or value below, and this calculator will show your current gestational age, estimated due date, trimester, conception date, pregnancy progress, and key milestone dates.
Which value do you know? Pick the one piece of information you already have. You can choose from your last menstrual period (LMP), your current gestational age, a gestational age from a past appointment, your conception date, your estimated due date, or a due date given by ultrasound or your doctor.
Enter Known Value: Type in the date or gestational age that matches your choice above. If you picked LMP, enter the first day of your last period. If you picked a gestational age option, enter the weeks and days. If you picked a due date or conception date, enter that date.
Second Dating Source (optional): If you want to compare two methods side by side, add a scan-based due date or an LMP date here. The calculator will show both gestational ages. Add the date of the ultrasound as well, and it will flag a difference large enough for ACOG to advise changing the due date for a scan at that stage.
Calculate gestational age as of: This is the date you want results for. It defaults to today, but you can change it to any past or future date to see what the gestational age will be on that day.
Menstrual Cycle Length: Enter your usual cycle length in days. The standard is 28 days. If your cycle is shorter or longer and you start from your LMP, the calculator moves the due date to match and counts gestational age back from it.
Press Calculate to see your full results, including a step-by-step breakdown, a pregnancy progress bar, a clinical milestone timeline chart, and a table of important screening and appointment windows. Press Reset to clear all fields and start over.
What Is Gestational Age?
Gestational age is how far along a pregnancy is. It is counted in weeks and days, starting from the first day of the mother's last menstrual period (LMP). This means gestational age starts about two weeks before the baby is actually conceived. By convention the due date is 280 days after the first day of the last period, a count that assumes a 28-day cycle with ovulation on day 14.1 That is 40 weeks.
How Is Gestational Age Calculated?
The most common way to find gestational age is to count the number of days from the first day of your last period to today. That total is then split into weeks and days. For example, if 145 days have passed since your LMP, your gestational age is 20 weeks and 5 days.
Doctors can also figure out gestational age using an ultrasound scan, a known conception date, or an estimated due date. If you start from your LMP and your cycle is longer or shorter than the standard 28 days, the calculator moves the due date by the difference. Gestational age is always counted back from the due date. Only about half of women recall the first day of their last menstrual period exactly.1
Why Does Gestational Age Matter?
Gestational age helps doctors track a baby's growth and schedule important tests and checkups. It tells them which trimester you are in, when to order screenings, and when the baby is expected to arrive. Knowing the correct gestational age is one of the most important parts of prenatal care.
Trimesters of Pregnancy
Pregnancy is split into three trimesters based on gestational age:
- First Trimester: Week 0 through Week 13. The baby's organs begin to form during this stage.
- Second Trimester: Week 14 through Week 27. The baby grows quickly, and the mother usually starts to feel movement.
- Third Trimester: Week 28 until birth. The baby gains weight and gets ready for delivery.
These follow ACOG's trimester boundaries: the first trimester runs to 13 weeks and 6 days, the second from 14 weeks and 0 days to 27 weeks and 6 days, and the third from 28 weeks and 0 days.2
Estimated Due Date (EDD)
The estimated due date is the day your baby is expected to be born. It is calculated by adding 280 days to the first day of your last period.1 Few babies arrive on the day itself; ACOG calls a pregnancy of 41 to 42 weeks late term and one beyond 42 weeks postterm.3
LMP vs. Ultrasound Dating
Sometimes the gestational age from an ultrasound does not match the age based on your LMP. Small differences of a few days are normal. ACOG calls first-trimester ultrasound the most accurate dating method.1 It advises changing the due date when the scan and the LMP date differ by more than 5 days for a scan before 9 weeks, more than 7 days for a scan from 9 weeks to 15 weeks and 6 days, more than 10 days from 16 weeks to 21 weeks and 6 days, more than 14 days from 22 weeks to 27 weeks and 6 days, and more than 21 days when the first scan is at 28 weeks or later.1 These limits apply to the scan that first sets the due date. Once a due date has been selected, it does not change with later ultrasounds.3