How many weeks pregnant am I?

Many IVF patients get very confused about how the doctor calculates the age of their pregnancy (= gestational age, in medical jargon). Logically, shouldn't it be from the day of the embryo transfer? After all, it's only after the embryos are transferred that a woman can be considered to be pregnant!

However, doctors are not always logical, and we usually use the menstrual age when talking about the length of the pregnancy. This is because obstetricians usually see women who have got pregnant after having sex in their bedroom. Very few of them will know the exact date they ovulated, which is why we use the menstrual age in clinical practice. This does not change just because you have had an IVF pregnancy - the clinical rules remain the same!

This creates a lot of confusion in patient's minds - especially when they are trying to make sense of their ultrasound scan results or their HCG levels.

Remember that your OB is always talking about the menstrual age - not the age of theembryo ! This is purely for clinical convenience and is a well-accepted universal convention.

So how do you convert the date of embryo transfer to menstrual age. This is very simple! The key reproductive event you need to focus on is ovulation! It makes much more sense to talk about the pregnancy in terms of DPO (days post ovulation), rather than the menstrual age or the day of the embryo transfer. This is because we can use this landmark for all situations (including IUI pregnancies; and for Day 3 embryo transfers and Day 5 embryo transfers aswell !)

Since every IVF patient knows the date they ovulated (= the day of egg collection), it's easy to calculate your menstrual age. Just subtract 14 from your date of ovulation. This is your "corrected LMP" (last menstrual period).

(Corrected) LMP = Date of egg collection minus 14

The reason we do this is simple - it's because text books assume the follicular phase is exactly 14 days! Once you know your corrected LMP, its then easy to use this as the anchor, based on which your OB can calculate your gestational age.

This means that the menstrual age will always be 14 days more than the actual age of the embryo.

Confused? Let's look at an example.

Let's suppose your LMP was 5 Jan; and your egg collection was done on 24 Jan (let’s assume you took a long time to grow eggs); and your embryo transfer was done on 29 Jan (Day 5 transfer). The HCG pregnancy test will be usually done about 14 days post ovulation (DPO), which is 7 Feb. If it's positive - say 120mIU/ml, the doctor will confirm you are pregnant! This means that even though you are only 14 DPO ( and your embryo's age is only 14 days ) , he will calculate your corrected LMP as 10 Jan ( date of ovulation , 24 Jan, minus 14 days) - which means your clinical gestational age ( or menstrual age) will become 4 weeks ( 28 days) ! Once you understand this “2 week gap" and the rationale behind it, you'll find it much easier to date your pregnancy!