How Accurate is Your Due Date?

Statistics show that only 4% of babies arrive on their due date. Of the 96% of babies who aren’t born on their due date, 30% early and 70% late. Babies arriving early can catch women off guard, while babies arriving 2 weeks after their due date can result in very uncomfortable, restless mothers-to-be! So why are so few babies born on their due date?

Reasons for Inaccuracy
One reason only 4% of women deliver on their due dates is because the method used by doctors to work due dates isn’t 100% accurate. Your due date is calculated by working out the first day of your last period, and then adding on 280 days (40 weeks) to get your due date. This method assumes that every woman’s menstrual cycle lasts 28 days, as this is the average length of time for menstrual cycles. However, if like many women, your menstrual cycles are longer than this, you’re more likely to deliver after your due date. Similarly, women who have shorter cycles are likely to deliver early. Another factor affecting the accuracy of due dates is that some women aren’t sure of the date of their last period. If you have to guess when it was, your due date could be out by weeks.

Ultrasound scans are used at 20 weeks and then again at 28 weeks in the UK. These can give a much clearer idea of a due date, if a woman is unsure of the date of her last period. If a scan is done in the first 3 months, it can calculate fetal age within a 3-5 day range either side. If a scan is done in the second trimester, it is usually accurate to within 7-10 days, but by the third trimester a scan can be inaccurate by up to 3 weeks either side.

Are There Any Other Factors?
Aside from the inaccuracy of the methods used to determine a due date, there are many other factors that affect when a baby will be born. These include the mother’s health, age, weight, whether she is a first time mother, whether she smokes, previous complications during pregnancy, and prenatal nutrition. Women who smoke or drink heavily, or use drugs during pregnancy are more likely to give birth before their due date, as are overweight mothers, women under 20 and over 45, and women expecting twins or triplets. Studies have also found that women who feel stressed or suffer depression in their pregnancies are more likely to give birth before their due date.

Several medical conditions also affect the date of delivery, such as gestational diabetes, pre-eclampsia, or if the placenta stops working. These are rare but serious conditions, so make sure you know what symptoms to look out for. The high rate of babies born by caesarean is another reason why due dates aren’t accurate, as planned caesareans are often performed about 2 weeks before a woman’s due date.

Even Pregnancy isn’t Accurate!
Even if you know the exact date your baby was conceived, your due date might still not be accurate as a normal pregnancy can be anything between 37-42 weeks. 40 weeks is used as your due date as this is the average length of pregnancy.

Due dates are meant to used as a rough guide, based on the average length of pregnancy. It’s best to expect that your baby could come anytime after 36 weeks, if your pregnancy is normal, so be prepared for your baby arriving earlier than your due date by packing your hospital bag, making sure you’ve bought a car seat to get the baby home, buying a moses basket or cot for your home, and any other essentials you’ll need for those wonderful, hectic first few weeks!

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This article has been written by Alice from and she wants to make sure that people aren’t surprised if their baby turns up early or late!

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