How to calculate the weeks of pregnancy if I have irregular cycles?

I am pregnant but have an irregular cycle.

How can I trace the weeks of pregnancy?

This is a very common question, so we try to give you a clear and comprehensive explanation.

The calculators that you find on the net determine the weeks of pregnancy by asking you to enter only the date of the last period.

They are therefore based on cycles of approximately 28 days with ovulation occurring approximately 14 days after the start of menstruation.

Perfect cycles of this type are uncommon, so to better calculate my gestational age, it’s always good to know when ovulation took place.

Knowing exactly or approximately the date of ovulation I can determine the gestational age more accurately, before having confirmation with an ultrasound.

How to calculate the week of pregnancy knowing the day of ovulation

If I’m irregular but I know when I ovulated it’s all very simple.

I can know when I ovulated thanks to the basal temperature graphs that allow me to trace the day of ovulation and conception with good accuracy.

See graphs of basal temperature of pregnancies

If I don’t track ovulation with graphs but use ovulation tests, I consider the day after the positive test (or the day after the last of the last positive tests) as the ovulation date.

If I don’t fill in the graphs and I don’t do the ovulation tests but I simply know that I have ovulated thanks to the classic symptoms that recur month after month and that I mark on the calendar, use this date.

Once the ovulation date has been identified, which we will call day X, I will go backwards in the calendar of 14 days starting from day X and the 14th day before ovulation will be my fictitious last menstruation date, which I will use to calculate the weeks of pregnancy .

Example :

I know I ovulated on November 20th, I put myself on the calendar, start on November 20th and go back 14 days. I arrive on November 7th and this will be the date that I will use as the last period date, in the pregnancy weeks calculator.

If I don’t know the date of ovulation and last menstruation

In this case it is difficult to have an idea of ​​the weeks of pregnancy before the first ultrasound which takes place around 8-9 weeks of pregnancy (or in any case within the first three months), which, being called for dating, manages to date with good approximate gestational age.

You can obviously do beta HCG but as you see in this diagram

Beta Beta HCG values ​​by week of pregnancy

The reference values ​​are very wide and therefore you can easily go wrong by one or two weeks…

If you have had a single risky relationship, everything is easier, because in this case conception can occur on the day of intercourse or within the following 5 days (due to the survival of the sperm).

It can help me to remember when I had the intercourse that led to conception, if I only had one…

Knowing the moment of conception for the relationship had then you can use the procedure that is used when you know the moment of ovulation, explained in the previous chapter.

Obviously, as already mentioned, it will then be the first ultrasound to date the pregnancy with precision when the data available are uncertain.

Why it is important to know the gestational age

First of all it is important that pregnancy is not defined in months but in weeks.

We explained the reason here:

They are called weeks of pregnancy, just talk about months

Measuring a pregnancy in months is grossly medically inaccurate.

Knowing the weeks of pregnancy allows you to correctly book medical visits, to take the tests scheduled in the correct weeks, to prepare the documentation for maternity with the right deadlines …

It also allows you to follow the evolution of the pregnancy step by step (see: pregnancy week by week, how your body changes and how your baby grows )

And last but not least, knowing correctly when conception occurred allows you to better calculate the due date. This calculation always uses the last menstruation date, which should be determined as explained above if the cycle is irregular.