IVF Pregnancy Calculator and the 2 week gap – When is My Baby due ?

The moment you find out you are pregnant, the next question is – when is my baby due ? This is true, whether you get pregnant in your bedroom, or after doing IVF !

There are lots of pregnancy calculators online , which allow women to calculate their due date. The principle is quite simple – they just need to key in their LMP ( first day of their last menstrual period) and the calculator will tell them their due date ( by adding 40 weeks do the LMP, a rule of thumb called Naegle’s rule).

One would think that calculating the due date after IVF treatment should be much easier, because we know the exact date on which the eggs were retrieved and the embryos were transferred ! However, it can be more confusing, because of the difference between menstrual age and embryonic age. This is why many couples feel lost. Doctors often make matter worse, especially when they do HCG tests and ultrasound scans. Too much data can confuse the poor patient, because you just want to know when your bundle of joy will be with you. 

Basics of Your Due Date

Your due date is usually calculated based on a 40 week average gestational age. This is an estimate , calculated from the first date of the last menstrual period. Now we know that a woman cannot not pregnant before she ovulates – and ovulation , fertilization and conception occur about 14 days from the start of the menstrual cycle. Therefore, if a woman with a 28-day cycle had her last period on January 1, her due date is 8 Oct . However, the baby was probably conceived around January 14. This means on 1 Feb, while the embryo’s age is only 4 weeks, the doctor will say she is 4 weeks pregnant !.   This is a clinical norm we use, which refers to the menstrual age, and not the actual age of the fetus. The reason for this is that most women don’t know on which date they ovulated; or on which day they had sex which led them to get pregnant. However, since the vast majority know their menstrual cycle well, it’s easy and convenient to use menstrual age for dating the pregnancy in clinical practice , in order to simplify matters. 

Calculating IVF Birth Date

When you do IVF, you do not need to use the date of  your last menstrual period to calculate your due date – you have far more information , which is much more biologically reliable.  The trouble is that if you calculate your gestational age from the date of the embryo transfer or the date of fertilization, your obstetrician will get confused, because he is used to calculating the age of the pregnancy in menstrual weeks – not the embryonic age. This means that even though you are actually being more scientifically accurate, you will have to follow the norms which clinicians use.

So how do you convert the date of your embryo transfer to menstrual age ? This is surprisingly 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; for Day 3 embryo transfers and Day 5 embryo transfers ; and frozen cycles as well !)

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) . This means ( 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, it's 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 120 mIU/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 !

The way labs report HCG results just makes matters worse ! If you remember the above rules, it’ll be much easier for you to make sense of your HCG results and date your pregnancy.

Ultrasound scans are another minefield. While the sonographer can measure the baby very precisely, and the machine uses a computer to calculate the age of the baby ( and based on this, it reports your expected due date), please do remember that even though the ultrasound machine uses a computer to do its calculations, these are only estimates, and not as precise as the date you calculate based on the date of egg collection !
 Ultrasound measurements cannot be taken as standard , because embryos grow at different rates.

Do also remember that babies born between 37 to 42 weeks from the last menstrual period are also considered to be full term.  In real life , only a small number of women give birth precisely on their calculated due date. If you have a multiple pregnancy, the chances of reaching the magic due date go down quite a bit.