How Many Weeks Pregnant Calculator
Work out exactly how far along you are. Enter your last period or your due date to find your pregnancy week, trimester, and estimated due date in seconds.
Find Out How Far Along You Are
Choose whether to calculate from the first day of your last period or from a due date you already know, then enter the date below for an instant estimate.
Your Details
All dates are used only in your browser and are never stored
Your Estimated Result
Based on the date you entered
How Pregnancy Dating Works
Pregnancy dates can feel confusing at first. Here’s how the standard 40-week system is worked out and what happens at each stage.
From Last Period to Due Date
The standard method used by midwives and doctors
Dating Starts From Your Last Period
Rather than counting from conception, pregnancy is measured from the first day of your last menstrual period (LMP), since ovulation dates are usually harder to pinpoint than period dates.
Ovulation & Conception
Conception typically occurs around two weeks after the LMP, meaning you’re already considered “2 weeks pregnant” at the point of conception for an average 28-day cycle.
The 40-Week Estimate
A full-term pregnancy is estimated at 40 weeks (280 days) from the LMP. This is an average — babies are considered full term from 37 weeks and can arrive anytime from around 37 to 42 weeks.
Dating Scan Adjustment
An ultrasound scan, usually between 8 and 14 weeks, measures the baby to confirm or adjust the estimated due date, since it’s often more accurate than LMP-based dating alone, especially for irregular cycles.
Tracking Progress by Trimester
Pregnancy is split into three trimesters, each roughly 13 weeks long, used by healthcare providers to group typical development stages and recommended check-ups.
Early Pregnancy Checklist
Steps to take once you know your estimated dates
Trimester Breakdown
A quick summary of when each trimester starts and ends, and the typical milestone associated with it.
| Trimester | Weeks | Typical Milestone |
|---|---|---|
| First Trimester | Week 1 to Week 12 | Booking appointment & early scan |
| Second Trimester | Week 13 to Week 26 | Anomaly scan (~20 weeks) |
| Third Trimester | Week 27 to Week 40 | Birth planning & final check-ups |
| Full Term | Week 37 to Week 42 | Baby is considered full term |
Pregnancy Dating FAQ
Answers to the most frequently asked questions about how pregnancy weeks and due dates are worked out.
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the date of conception. This is why a full-term pregnancy is described as 40 weeks, even though conception typically happens around two weeks after the LMP.
The standard method, known as Naegele’s rule, adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your average cycle length. An ultrasound dating scan, usually performed between 8 and 14 weeks, often gives a more accurate estimate and may adjust your due date slightly.
The first trimester runs from week 1 to the end of week 12. The second trimester runs from week 13 to the end of week 26. The third trimester runs from week 27 until birth, typically around week 40.
No. It gives an estimated conception date based on typical ovulation timing, usually around 14 days after the start of your last period for an average 28-day cycle. Actual ovulation and conception can vary, so this figure is an estimate rather than a precise date.