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How to date your pregnancy if you know when you ovulated

How to date your pregnancy if you know when you ovulated

November 25, 2022

How to date your pregnancy if you know when you ovulated

“My doctor will only use the last day of my period to date my pregnancy,  but I have irregular cycles. This means the doctor thinks I'm almost 4 weeks longer into my pregnancy than I actually am.” 


“I had to lie to my doctor about when my last period was, with my last pregnancy they didn’t trust my own ovulation tracking and tried to induce my baby too early. I don’t want to risk that again”


“I had to wait for my baby to be measured at the first scan before the midwife would believe that I was only a few weeks into my pregnancy and not the 8 weeks they predicted.”  

Does any of this sound familiar to you? Is this your experience or that of someone you know too?

Let’s start with why charting can help you not only to conceive, but also for you to advocate for yourself in the doctor’s office. 

Pregnant woman

The first step when dating a pregnancy is to determine the first day of your last menstrual period. Next the doctor will count back 3 calendar months from that date and then add 1 year and 7 days to that date, or add 40 weeks (many offices have a system that does this automatically). Unfortunately, though, this is based on a typical 28 day cycle, with ovulation assumed on day 14 of that 28-day cycle. If your pregnancy is only based on last menstrual period, this can lead to potentially being inducted sooner than necessary.

This is particularly a problem if you have irregular cycles (irregular ovulation). Working from the typical day 14 ovulation date will be incorrect, perhaps you don’t ovulate until day 40, or even later than that. If you have at least one ultrasound, it is likely that your provider will date your pregnancy based on an ultrasound instead. However, if you decline ultrasounds, don’t have an early ultrasound, or they find it difficult to get a good measurement, your temperature may officially be ahead of when it should be based off of ovulation. Tracking your basal body temperature (BBT) and cervical mucus will help you to pinpoint as close as possible the time of ovulation and confirm a more accurate due date for your pregnancy.

This data can be easily shared with your doctor, with the explanation of when ovulation took place to a more accurate date. Based on this information, they can avoid applying the typical ovulation-on-day-14 cycle rules.

But what if your physician doesn’t acknowledge your personal charting data? 

Obstetrician checkup for pregnant woman

There are times, however, when doctors refuse to use the information, which is the case with some of our Tempdrop users. There are a couple of options available to you. The first option is to find another physician who will take into account the knowledge you have about your own body and respect your wishes. Experience has shown that there are many other doctors who are more than comfortable to use charting data to accurately date pregnancy and ensure a smooth pregnancy. 

Alternatively, in most European countries you can request that a revision is added to your medical notes. You can also wait for your first ultrasound appointment to officially revise the dates, based on the measurements of your baby. In the US, the majority of providers will adjust your due date if your early ultrasound is different from your due date based on the last menstrual period if the difference is more than 4-7 days (depending on the office).

How to date a pregnancy with ovulation.

If you believe or know that your provider won’t date your pregnancy based off of your ovulation date, then you can find it yourself! To calculate your due date, add 38 weeks to your presumed ovulation day! Since providers use the first day of the last menstrual period (textbook 2 weeks before ovulation), it’s only 38 weeks rather than 40 from ovulation day. This is easy to do, as you can search “38 weeks from [date]” on Google and it will give you an answer! If your doctor asks for your last menstrual period, give them the date 2 weeks prior to ovulation.

We hope that this situation doesn’t occur, or that it’s something you can discuss with your provider. Either way, tracking your cycle will enable you to feel empowered and to be able to clearly advocate for your own health and that of your unborn baby. 


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If you would like to learn more about the benefits of tracking BBT and cervical mucus and how to do it, then visit our resources page.

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