Pregnancy Week by Week Calculator – Due Date, Current Week and Trimesters
Pregnancy Calculator
Calculate your estimated due date, track your current week, and see your trimester milestones based on your last menstrual period (LMP).
Quick answer
A pregnancy week by week calculator estimates your due date and tells you how many weeks and days pregnant you are. Choose whether you know the first day of your last period or the date of conception, enter the date and your average cycle length, and click Calculate Due Date. You will see your estimated due date, a progress bar to week 40 and the dates for each trimester.
Our free pregnancy week by week calculator uses Naegele’s rule, the method most doctors and midwives use for an initial due date: 280 days (40 weeks) from the first day of your last menstrual period, adjusted for cycles that are longer or shorter than 28 days. If you know your conception date, it adds 266 days instead.
A due date is an estimate. Only around 4 to 5% of babies arrive on the exact day, and most are born within two weeks either side. An early ultrasound scan usually gives the most accurate dating, so always follow the date your healthcare provider gives you.
How the Pregnancy Week by Week Calculator Works
- From your last period (LMP): due date = first day of last period + 280 days, plus or minus the difference between your cycle length and 28 days.
- From conception: due date = conception date + 266 days.
- Current week: days since the LMP divided by 7, shown as weeks and days.
Example: if your last period started on 10 June 2026 and your cycle is 28 days, your estimated due date is 17 March 2027, and on 7 October 2026 you are 17 weeks and 0 days pregnant.
Pregnancy Trimesters at a Glance
| Trimester | Weeks | Common milestones |
|---|---|---|
| First | Weeks 1 to 13 | Positive test, first antenatal visit, dating scan, early nausea and tiredness |
| Second | Weeks 14 to 27 | Anatomy scan around 18 to 22 weeks, first movements, energy often returns |
| Third | Weeks 28 to 40 | Baby gains weight, more frequent check-ups, birth planning |
How to Use the Pregnancy Week by Week Calculator
Follow these steps to use the pregnancy week by week calculator.
Step 1: Choose Last Period or Conception Date
In Calculation Method, select First Day of Last Period (LMP) if you know when your last period began, or Exact Date of Conception if you know when you conceived.
Step 2: Enter the Date and Your Cycle Length
Pick the date in the calendar and choose your average cycle length. Leave it at 28 days if you are not sure.
Step 3: See Your Due Date and How Far Along You Are
Click Calculate Due Date. The result shows your estimated due date, how many weeks and days pregnant you are and how many days are left.
Step 4: Check Your Trimester Milestones
Below the due date, the calculator lists when your first trimester ends and when your second and third trimesters begin.
Why Cycle Length Matters
Naegele’s rule assumes ovulation about 14 days after your period starts, which fits a 28-day cycle. If your cycle is 32 days, you probably ovulated about four days later, so the pregnancy week by week calculator moves your due date four days later too. A 24-day cycle moves it about four days earlier.
When to Talk to Your Healthcare Provider
- As soon as you have a positive pregnancy test, to book your first appointment.
- If you have heavy bleeding, severe pain, fainting or a high temperature.
- If you notice reduced baby movements later in pregnancy.
- If your due date from a scan is different from this calculator; the scan date usually takes priority.
Why Use a Pregnancy Week by Week Calculator?
Knowing your week helps you understand which tests are coming up, what changes to expect and how your baby is developing. A pregnancy week by week calculator gives you the date, your current week and your trimester dates in one place, so you can plan appointments, leave from work and preparations with less guesswork.
Frequently Asked Questions
Is this pregnancy week by week calculator free?
Yes. It is free, private and needs no sign-up.
How is the due date calculated?
It adds 280 days to the first day of your last period, adjusted for your cycle length, or 266 days to the conception date.
How accurate is a due date?
It is an estimate. Most babies are born within two weeks either side of the due date, and only around 4 to 5% arrive on the day itself.
When does each trimester start?
The first trimester covers weeks 1 to 13, the second weeks 14 to 27 and the third weeks 28 to 40.
Why is pregnancy counted from my last period?
Conception date is often unknown, so pregnancy is counted from the first day of the last period, about two weeks before conception.
Does this replace my doctor’s due date?
No. Always follow the date given by your doctor or midwife, especially after a dating scan.