Pregnancy & Due Date Calculator
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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. 2 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. It is for this reason that calculations using Naegele’s rule can be considered as an estimate and not a definitive date. 3
How far along am I?
To calculate how many weeks pregnant you are, our pregnancy calculator looks at the first day of your last menstrual period (LMP) and compares it to today's date. 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. If your cycle is different to 28 days, your calculation will vary based upon this.
You will find that the sonographer at your first ultrasound scan (often called the dating scan) will give you a more accurate idea of how many weeks pregnant you are by measuring your baby from its head to its bottom to form a determination.
If you want to calculate an estimated due date manually using Naegele’s rule, here's how to do it:
- Find out the first day of your last menstrual period
- Subtract 3 months from that date
- Add 1 year and 7 days
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. 1
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+
You can read more about the three trimesters in this article about the stages of pregnancy.
When will my baby be born?
This pregnancy calculator uses one of two methods to work out when your baby might be born. If you fill in the start date of your last period, it takes that date and adds 40 weeks (assuming you've chosen a 28 day average cycle). If you've chosen a different cycle, it will add or subtract days based upon that. If you choose to calculate by ultrasound date, the calculator will work out your due date based upon how far along you are on that date. This will likely yield a more accurate result.
The 280 day figure is based upon Naegele’s Rule and is the most common method of calculating due dates. German obstetrician Franz Naegele created his formula in 1812 and it has been referenced ever since.
How accurate is this pregnancy calculator?
It is important to remember that this pregnancy calculator is meant to be a bit of fun and the results contain guesswork (as with all 'predictions' about the future). Assuming a natural birth, there's no way of knowing for certain when your baby will arrive. It is worth noting that only 5% of babies arrive on their due date. 1 The vast majority are born two weeks either side. This calculator, as with all due date calculators, should be used only as a guide, to give you a little help with planning for your baby's arrival.
At the point of 42 weeks, births are commonly considered 'post-term'. If you reach this point, your hospital will likely induce labour for the safety of the baby. You can learn more about this and about due dates in this article by the Essential Parent website .
How do the due date charts work?
The due date charts show possible due dates based upon data from a birth survey carried out by Spacefem.com.4 The survey relied upon correct information being input by each of the 9,000+ Mums who took part; it was not a peer reviewed scientific study. It's important to remember that your birth date probability will vary based upon many factors, including your age, health, ethnic origin and whether you've given birth before.Calculator created by Alastair Hazell. Reviewed by Aaron Kandola, Ph.D.
- Edwards KI, Itzhak P. (Updated 2020 Nov 6). Estimated Date of Delivery. https://www.ncbi.nlm.nih.gov/books/NBK536986/
- AIMS. (2020). How accurate is my 'due date'?. https://www.aims.org.uk/information/item/due-date
- International Journal of Obstetrics & Gynaecology. Naegele’s rule and the length of pregnancy. https://obgyn.onlinelibrary.wiley.com/doi/10.1111/ajo.13253
- SpaceFEM due date study (not peer reviewed). Spacefem.com
If you have any problems using our pregnancy calculator, please contact us.
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