Updated: 28/09/2020

Due Date Calculator

When you know that you’re pregnant, one of the other things you’ll want to know is when your baby is due to be born. It’s an exciting time, and you’ll probably want to know how the rest of your year is going to be. You probably don’t want to wait until your first appointment to find out, so here’s how to calculate your due date. 

 

How To Calculate Your Due Date

There’s a few ways to calculate your due date. Some of these methods will need to be carried out by your midwife, or doctor. 

 

Conception Date: This is only possible if you know when you conceived. If you know that date, you can add 266 days to the conception date to get your due date. 

First Day Of Your Last Period: There’s a few methods to using the date of the first day of your last period to get your due date. The first is to count 280 days from the date of the first day of your last period. 

The second is to go back three months from the first day of your last period, and add seven days. As an example, if the first day of your period was May 10, go back three months to February 10. Then add seven days, so your due date would be February 17. 

If math isn’t your strong suit, then you can use an online due date calculator. They work on the same principles as above, but they do all the counting for you. You could try the NHS due date calculator, or yourduedate.

The third method is the Nichols method. It changes depending on if this is your first pregnancy, or not. For your first pregnancy, you take the start date of your last period, add 1 year, then take away 2 months and 2 weeks, and then add, or take away the number of days difference between your cycle and a 28 day cycle. 

If this is your third, or any subsequent pregnancy, take the date of your last period, add 1 year, then take away 2 months, and 2 and a half weeks, then add or take away the number of days difference between your cycle and a 28 day cycle. 

If you normally have 35 days between your periods then you would add 7 days. If you normally have 21 days between your periods then you would take away 7 days.

It sounds complicated but it can give you a more accurate due date, as it accounts for menstrual cycles that are longer or shorter than 28 days. 

The fourth method is a calculation devised by Mittendorf et al. It also changes depending on if it’s your first pregnancy or not. 

In the first pregnancy, take the date of your last period, take away 3 months, then add 15 days. If this is your third pregnancy, or any subsequent pregnancy, take the date of your last period, take away 3 months, and then add 10 days. 

Heartbeat: Your healthcare provider will have the equipment needed to listen to your baby’s heartbeat. Some midwives use a stethoscope, others will use a device called a doppler. You can buy a doppler to use at home, but it’s best to let the midwife do this if you’re using the heart beat to calculate the due date. The heartbeat can be first heard at around 9 or 10 weeks of pregnancy. 

Foetal Movement: Judging how many weeks pregnant you are based on the foetal movement is not an exact science. It can give you a rough idea, but your healthcare provider will carry out other tests and checks to confirm. You should begin to feel your baby move at 18 to 22 weeks. It may feel a little like having butterflies in your stomach. 

Ultrasound: Some women are offered an early ultrasound to check the baby over. Some clinics offer early ultrasounds routinely, but others will only do so in certain criteria. An early ultrasound can give a more accurate estimate of your due date, as it will be based on the baby’s size and development. 

 

Are Due Dates Accurate?

The short answer is not really. Only around 5% of women actually give birth on their due date. There’s so many variables that it’s almost impossible to give a very accurate due date. Things like the length of your menstrual cycle, your ovulation dates, and your baby’s development can all impact when you give birth, and make it very difficult to predict. 

Instead, many obstetricians recommend that you see your due date as a very rough guide. You could go into labour during the weeks surrounding your due date.

Why Your Due Date Can Change

Ultrasounds: Although your healthcare provider will give you a rough due date at your first appointment, this can be changed at your first ultrasound. Dating during your ultrasound will be based on the baby’s growth measurements. During the first trimester of pregnancy, all babies tend to have the same growth rate. This means that a due date based on the first scan is likely to be fairly realistic, although in no way is it a guarantee. 

During the second, and third trimesters, growth can vary hugely. No pregnancy is the same, so no baby grows in the same way, at the same rate during these months. This means that there’s no reliable way to give an accurate due date from scans during this time. 

Fundal Height: It’s possible that you’ve never heard the words fundal height before. This is a measurement of the distance between the top of your pubic bone, and the top of your uterus. It measures the size of your womb, and indicates how far along your pregnancy is. If the measurement is different from standard growth charts by around three weeks, your healthcare provider may change your due date. 

Blood Tests: Your healthcare provider will take routine blood tests through your pregnancy. Sometimes the results of the blood tests can indicate that your pregnancy is further along, or not as advanced as initially thought. Your due date may be changed to reflect this.

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