What is my due date?
Your due date, also known as your estimated date of delivery, is the approximate date your labor might begin. Your estimated due date is commonly calculated by adding 280 days (40 weeks) to the date of the first day of your last period, using Naegele's Rule. This may vary if you have irregular periods.
It's worth noting that although predicted due dates are commonly calculated at 280 days, a baby is born on its predicted due date just 5% of the time. A 2020 study published in the International Journal of Obstetrics & Gynaecology suggests a number of factors that can influence the length of your pregnancy, including ethnicity, height, variations in the menstrual cycle, the timing of ovulation, parity and maternal weight.
How to calculate due date manually
If you want to calculate an estimated due date manually using Naegele's Rule, here's how:
- Find out the first day of your last menstrual period
- Subtract 3 months from that date
- Add 1 year and 7 days
Example
If your last period started on January 1, 2026:
- Subtract 3 months → October 1, 2025
- Add 1 year and 7 days → October 8, 2026
How far along am I?
To calculate how many weeks pregnant you are, count from the first day of your last menstrual period (LMP). The first day of your LMP counts as the first day of your pregnancy. This calculation is based upon an average cycle of 28 days.
The sonographer at your first ultrasound scan (the dating scan) will give you a more accurate idea by measuring your baby's size.
How many weeks are you pregnant for?
Around 90% of pregnancies last between 37 and 42 weeks. If a baby is born before 37 weeks, he/she is considered preterm or premature.
The three trimesters
There are three trimesters in a pregnancy, with around 13 or 14 weeks per trimester:
- First trimester: Weeks 0 to 12
- Second trimester: Weeks 13 to 27
- Third trimester: Weeks 28 to 40+
First Trimester (Weeks 0–12)
The first trimester is when major organs and body systems form. You may experience morning sickness, fatigue, and tender breasts.
Second Trimester (Weeks 13–27)
The second trimester is often called the "honeymoon period" of pregnancy. Many women feel more energetic and nausea typically subsides.
Third Trimester (Weeks 28–40+)
The third trimester is when your baby gains weight rapidly and prepares for delivery. Regular checkups become more frequent.
Birth probability by week
Based on survey data of over 9,000 mothers, here is the approximate probability of giving birth during each week:
| Week | Probability |
|---|---|
| Week 35 | 1.5% |
| Week 36 | 3.5% |
| Week 37 | 8.0% |
| Week 38 | 15.5% |
| Week 39 | 25.0% |
| Week 40 | 27.0% |
| Week 41 | 15.0% |
| Week 42 | 4.5% |
Only about 5% of babies are born on their exact due date.
Factors that affect due date
Cycle Length
Naegele's Rule assumes a 28-day cycle. If your cycle is longer or shorter, your actual due date may differ by several days.
Ethnicity
Research has shown that gestational length can vary between ethnic groups.
Height and Weight
Maternal height and pre-pregnancy weight can influence the length of pregnancy.
Parity
Whether this is your first baby can affect the timing of delivery — first-time mothers are more likely to deliver slightly later.
References
- Edwards KI, Itzhak P. (2020). Estimated Date of Delivery. StatPearls. NCBI.
- AIMS. How accurate is my 'due date'?. aims.org.uk.
- International Journal of Obstetrics & Gynaecology. Naegele's rule and the length of pregnancy.
- SpaceFEM due date study (survey of 9,000+ mothers).