How the Pregnancy Due Date Calculator Works
A due date, also called an "estimated date of delivery" (EDD), tells you the approximate day your baby is expected to arrive. No calculator can predict the exact day of birth, but it gives you and your healthcare provider a reliable target to plan around, schedule prenatal checkups, and track your baby's growth week by week. This calculator supports three of the most common ways doctors and expecting parents estimate a due date: the last menstrual period, the conception date, and an ultrasound scan.
Method 1: Last Menstrual Period (LMP)
This is the method most doctors use first, known as Naegele's Rule. It assumes a fairly standard 28-day cycle and adds 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the calculator adjusts slightly to give a more personalized estimate, since ovulation timing shifts with cycle length.
Method 2: Conception Date
If you know roughly when conception happened, perhaps from tracking ovulation or fertility treatment, this method adds 266 days (38 weeks) to that date. It tends to be more precise than the LMP method for people with irregular cycles, since it skips the guesswork around ovulation timing entirely.
Method 3: Ultrasound Dating
Early ultrasounds, particularly those performed between weeks 8 and 14, are widely considered the most accurate way to date a pregnancy because fetal growth is very consistent in early weeks. If you enter the gestational age shown on your scan report along with the scan date, the calculator works backward to estimate your due date and current week automatically.
Understanding Your Results
Once you calculate, you will see your estimated due date, how many weeks and days pregnant you currently are, which trimester you're in, and how many weeks remain. The tool also maps out key milestones, including the estimated conception date, the end of the first trimester, the point of viability, and when full term begins.
Why Trimesters Matter
Pregnancy is typically divided into three trimesters. The first trimester (weeks 1-13) is when most of your baby's organs begin to form. The second trimester (weeks 14-27) is often called the most comfortable stage, as early symptoms ease and you may start feeling movement. The third trimester (weeks 28 until birth) is focused on rapid weight gain and final preparation for delivery.
Full Term, Early Term, and Post-Term
Medical guidelines in the USA, UK, and Canada define full term as 39 to 40 weeks. Babies born between 37 and 38 weeks and 6 days are considered "early term," while those arriving after 42 weeks are described as "post-term." Only around 1 in 20 babies is actually born on their exact due date, so these ranges matter more than the single date itself.
Frequently Asked Questions
How is my pregnancy due date calculated?
Most due dates are calculated using Naegele's Rule: your last menstrual period plus 280 days (40 weeks). If you know your conception date instead, your due date is roughly conception plus 266 days (38 weeks).
How accurate is a due date calculator?
It provides an estimate, not a guarantee. Only about 5% of babies are born on their exact due date. Most healthy pregnancies deliver anywhere between 37 and 42 weeks.
Can I calculate my due date from an ultrasound?
Yes. Enter the gestational age from your ultrasound report along with the scan date, and the calculator works backward to estimate your due date. Early scans, before 14 weeks, tend to be the most accurate.
What are the three trimesters of pregnancy?
The first trimester runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 until birth, typically around week 40.
How many weeks is a full-term pregnancy?
A full-term pregnancy is generally 39 to 40 weeks. Babies born from 37 weeks are considered early term, and those after 42 weeks are post-term.