Calculating When Your Baby is Due

Determining the date you will give birth is one of the first things you will want to find out once you know you are pregnant. Not only does it have many practical implications – when you will feel tired, when you may have morning sickness, or when you can’t work any more – but it also puts a bookend on the whole pregnancy and the time from which you will become a mother.

There are several ways to determine when your baby will be due. On average human pregnancies last 266 days or roughly 40 weeks (9 months). Other species have shorter or longer periods of pregnancy – larger mammals generally taking longer and smaller mammals a shorter span. Of course, nature often takes things into its own hands so your baby may be earlier or later than the expected due date which is in fact an average of all women. We are all different, our bodies have different physiologies and we have different family histories so don’t expect to be able to determine your estimated time of birth down to the last hour.

1. 
   

   Date of last menstrual cycle

   
   

   If you know when you had your last menstrual period you can calculate your approximate due date by adding 280 days to that date.

   
   

   This works because on average it takes 14 days from your last period to actual conception (14 + 266 = 280). It relies on you knowing your menstrual cycle and that cycle being regular (eg. every 28 days).

   
   


2. 
   

   Date of conception

   
   

   If you know when you actually had sex and were ovulating, you can add 266 days to that date to determine a due date. This only works if you know your own body’s ovulation cycle, the exact date you had sex, and there weren’t multiple days you had sex during your ovulation cycle.

   
   


3. 
   

   Ultrasound fetal measurements

   
   

   A doctor can use ultrasound to take measurements while the baby is inside your uterus. These measurements can relate to the length of the baby, the size of its head or its other dimensions. Each of these measurements all indicate the approximate age of the fetus (based on average growth rates of fetuses measured over time) and taken together can provide a single due date.

   
   

   This method is likely to be more accurate than the methods above. Performing the ultrasound may also indicate other factors such as the sex of the baby (from 16 weeks) or whether you have twins. Ultrasounds taken over a period of months allows you to see the development of the fetus.

   
   

Some of these methods require the involvement of your doctor or obstetrician. They will be able to help you if you need any assistance to calculate the due date and will also be able to discuss your own circumstances and the chances that your baby may arrive earlier or later than the predicted due date.