Pregnancy Week Calculator: Know Your Week, Trimester, and Due Date
You’re pregnant, and the first thing you want to know is how far along you are and when the baby is due. The pregnancy week calculator below works both out from a single date: the first day of your last period. It gives your current week and day, your trimester, and your estimated due date, using the standard method doctors use.
Pregnancy week calculator: how far along are you
Enter the first day of your last menstrual period (LMP) and the pregnancy week calculator shows your current week and day, which trimester you’re in, your estimated due date, and how many days are left.
How the pregnancy week calculator works
Pregnancy is dated from the first day of your last period, not from conception. That sounds odd, since you weren’t pregnant on that day, but it’s the standard because most people know their last period date, while the exact conception day is hard to pin down. A full-term pregnancy is counted as 40 weeks, or 280 days, from that first day. The calculator adds 280 days to your LMP to estimate the due date, and counts the days since your LMP to find your current week.
So if your last period started 10 weeks ago, you’re 10 weeks pregnant, and your due date is about 30 weeks away.
Understanding your trimesters
Pregnancy splits into three trimesters, and the pregnancy week calculator tells you which one you’re in.
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | Weeks 1 to 13 | Baby’s organs form; early symptoms like nausea |
| Second | Weeks 14 to 27 | Often the most comfortable stretch; the bump shows |
| Third | Weeks 28 to 40 | Baby grows fast; you prepare for birth |
Knowing your trimester helps you follow what’s normal for this stage and when your main checkups and tests fall.
A few things to know about your due date
Your due date is an estimate, not a deadline. Only about 1 in 20 babies actually arrive on the exact date. Most are born in the two weeks around it, anywhere from 38 to 42 weeks. So treat the due date as the middle of a window, not a fixed day.
A couple of points worth knowing:
- If your periods are irregular or you’re unsure of your last period date, the LMP estimate can be off. An early ultrasound dates the pregnancy more accurately, and your doctor may adjust your due date after a scan.
- If you know your conception date (for example from fertility treatment), the dating works a little differently. Your doctor can confirm the exact week.
Frequently asked questions
How does a pregnancy week calculator work?
It counts from the first day of your last period. It finds your current week by counting the days since then, and your due date by adding 280 days (40 weeks). This is the standard dating method doctors use when there’s no early ultrasound yet.
How many weeks pregnant am I?
You are as many weeks pregnant as the time since the first day of your last period. If that was 8 weeks ago, you are 8 weeks pregnant. The pregnancy week calculator gives the exact week and day from your LMP date.
Is the due date from a pregnancy week calculator accurate?
It’s a solid estimate, within a window. The calculation is standard, but only about 1 in 20 babies arrive on the exact date, and most come within two weeks either side. An early ultrasound gives the most accurate due date, so your doctor may adjust it.
What if I don’t know my last period date?
Then the LMP method can’t be precise. Give your best estimate for a rough idea, but ask for an early ultrasound, which dates the pregnancy by the baby’s size. Your doctor will set your official due date from the scan.
Can I use it if my periods are irregular?
You can, but the result is less reliable, because the 40-week count assumes a regular cycle. If your cycle is irregular, treat the number as approximate and confirm your dates with an early ultrasound at your first appointment.
