IVF Calculations: How To Calculate Pregnancy Weeks After IVF

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One of the first questions a parent-to-be will ask after discovering they are pregnant is ‘when is my baby due?’ And this is the same for IVF parents as it is for those who fall pregnant naturally. But while the due date for a natural pregnancy is based on the menstrual cycle, for IVF it is normally based on what is known as the ‘embryonic age’. 

Here’s all you need to know about IVF calculations and how to calculate pregnancy weeks after IVF. 

Due date based on menstrual age

The average gestation for a pregnancy is generally taken to be 280 days, or 40 weeks. For a natural pregnancy, the pregnancy weeks are taken from the start of a woman’s last period, assuming she ovulated 14 days later. This gives an estimated due date for the baby based on the menstrual age and is the guide most obstetricians work to. 

Due date based on embryonic age

However, when it comes to calculating the due date for a pregnancy that is the result of IVF treatment, things need to be done a little differently as you don’t always know the date of your last period. But what you do have is a more organised calendar in terms of ovulation, egg retrieval and implantation. And it is all of this information, plus the general recognition of a 280-day pregnancy gestation, that is used. 

Calculating a due date after IVF

As IVF – or in vitro fertilisation – occurs outside of the body and in a lab, it is actually possible to know exactly when the egg was fertilised, which can also be translated as the date a woman conceived. However, this is not the start date when calculating the due date of an IVF pregnancy.

Due dates for an IVF pregnancy are actually based on the date the embryo was transferred back into the uterus.  So, working with the 280-day gestation period, plus the 14-day ovulation period and the day you had the embryo transferred, you can work out your baby’s expected due date.

Here is an example: your embryo was transferred on day 5 after IVF, which means your baby’s due date will be 261 days later. 

There are also numerous online calculators which can help you work out your IVF pregnancy calendar. 

Due dates are not set in stone

As well as doing the calculations based on the embryonic age to work out your expected IVF pregnancy due date, your fertility clinic will also closely monitor your pregnancy to ensure the foetus is developing as it should and is reaching its key gestation phases.

However, when it comes to any pregnancy, the anticipated due date is not set in stone, and this also applies to IVF. Premature labour is also a risk of an IVF-assisted conception, particularly with a multiple pregnancy. And more sluggish levels of the hormone progesterone can also mean you may give birth earlier than your calculated due date. 

So, the take-away on IVF calculations is that they are intended as a formulated guide, and your fertility specialist will also work closely with you to assess and monitor your pregnancy’s progress, right up to the birth.

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