Calculating your due date enables you to chart your baby’s development. For health professionals, the due date (officially calculated during the 12-week ultrasound scan) provides the basis by which they measure growth. If the baby is not growing as expected, your midwife or doctor will be able to identify if there are any issues through additional monitoring. This might result in extra growth scans, which include checking all is well with the blood flow from the placenta to the baby. In the later stages of your pregnancy, these checks will help to establish whether intervention is recommended – e.g. to induce labour early. It’s worth noting that although babies' growth is measured based on national averages, many babies are just born little, and are perfectly healthy! This is often the case with parents who themselves were smaller than average at birth.

From a personal perspective, it’s nice to monitor your baby’s progression through the pregnancy by following along with a week-by-week pregnancy diary, for example, which tells you what you might experience at any given point in your pregnancy, and what’s going on with your baby. This not only gives you some ideas of what to be mindful of – e.g. diet, baby movements, etc. – but also helps with bonding as you ‘get to know’ your baby. And, of course, knowing when your baby is likely to arrive, also helps with all the practical preparations, like packing your hospital bag, writing a birth plan, and preparing your home.

Yes, your due date might change between your first booking-in appointment – when you’re working on the basis of dates you’ve given your midwife (the same dates as you put into this due date calculator) – and your 12-week ultrasound scan. This is because the ultrasound scan provides a more accurate picture of your baby’s development. When you give your LMP to your midwife, they work out your due date based on that, assuming you have ovulated in the middle of your cycle. Not everyone does, however. Some will ovulate early in the cycle, and some later. Having the ultrasound at 12 weeks is more accurate as the baby changes vastly from day-to-day and week-to-week in the first trimester. Your sonographer will undertake many measurements during this and the 20-week scan that help give a better timeline. 

If you’re actively trying to get pregnant, you can of course decide whether you want to try for a spring baby, a summer baby, etc. However, these things don’t always pan out as expected and some couples can be trying for a long time before they fall pregnant. Even if you do conceive as planned, your baby’s arrival is not guaranteed to fall exactly at 40 weeks. They come when they come. Getting your head around this is good practice for being new parents, who are also at the mercy of their little one’s schedule!