Why the due date CANNOT be calculated with the 1st day of the last cycle

Unfortunately there are many problems nowadays because the due date is not calculated correctly. To begin with I’d like to ask you a question: doesn’t it seem odd that we are taught to ask the doctor to tell us when we had sex so we find out when the baby was made?? 

We are told to track our cycle and write down somewhere when we are having our periods. It’s very important that we do that. But it’s at least as important to take notes when we are having sex – especially when we want to get pregnant. Why does nobody tell us? 

How can the doctor know when we had sex? 


When doctors calculate the due date they always assume that ovulation took place on day 14. But this is not the case in irregular cycles. So if we are having irregular cycles and if we haven’t even had sex on or around day 14, the baby cannot have been made on that day. And then the due date is not calculated correctly. 

Especially in the techno-medical model of maternity care a wrong calculated due date can have a tremendous impact on pregnancy, birth, postpartum, parenting – and life. 

If we ask women about their birth experience or maybe it even happened to us, the following scenario sounds familiar: 

When approaching the calculated due date the woman is longing for pregnancy to be over. Her day to day life became more and more difficult because her belly has gotten really big by now. And she can’t wait to have the baby in her arms. When the actual due date arrives the woman is told to go to the hospital to be checked. If nothing happens she has to go back every other day to be checked again. Care providers at the hospital are getting nervous. They start telling her that if still nothing happens they need to induce birth trying to do so with pharmaceutical drugs and if that doesn’t help either they need to do a cesarean. Hearing all that the woman is getting nervous as well. She loses trust in her body and doubts if she is capable to give birth. The woman is missing support by the people surrounding her. Instead of encouraging her that women are born to birth our society nowadays and the whole system makes women believe that we need the help of drugs and technology to be able to give birth. And if you start with one intervention often more interventions are needed. That’s why the cesarean rate in almost all western, developed countries that are primarily using the techno-medical model of maternity care is higher than 30 % (which means more than every 3rd woman has a c-section!). 

Correct due date with the help of ultrasound? 

One other way for doctors to cross-check the due date is to use ultrasound. But I again want to invite you to ask yourself a question: if we have a look at either a child or an adult, do we know how old they are just by knowing their (estimated!) height and weight? 

The very sad part is that a lot of times an incorrect due date leads to man-made induced prematurity. If ovulation has not taken place on day 14 but let’s say on day 35 (which sure can happen) and birth is induced on EDD + 10 it would actually be only week 39! This means that we are not trusting the baby to tell us when it’s ready to come to this world but instead we do something that can have a huge impact and severe consequences for both the mother and the baby. 

The duration of pregnancy - and other happenings in life 

And one more thing to think about is that the duration of pregnancy can vary just like other happenings in life. Do we all get our first period at the same age? Do all teenage boys get the change of voice at the same age? Do we all enter menopause at the same age? Or do all apples on a tree mature at the same time? Pregnancy doesn't always have to be exactly 40 weeks. It can be less and it can be even 42 weeks. 

If we want to calculate the due date, here is a method to correctly calculate it. For this method it is very helpful if you not only track your periods but also your cycle observing the cervical mucus and/or temperature

Pregnancy normally has a length between 260 and 268 days (+/- 8 days). 

That's the correct way to calculate the due date: 

To calculate the due date correctly you need to take 

  • the day with the peak of cervical mucus or 
  • the 2nd day before raise of temperature or 
  • the date of conception (based on records) ​​​​

minus 7 days, plus 9 months 


Day with peak of cervical mucus: February 15th 
February 15th - 7 days + 9 months = November 8th 

Birth can and ought to be such a wonderful even pleasurable and empowering experience preparing us for motherhood. But that’s almost impossible as long as we trust technology and the intellect more than nature and our intuition. 

That’s why it’s so important that us women get to know our body again, pass on the knowledge to other women and our daughters, take responsibility and trust in ourselves. 

Do you have questions? Or do you want to tell your birth story? Write me  

I'm looking forward to you! 


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