Estimating your due date after IVF.

After going through the roller coaster ride of IVF wondering whether it is going to be successful or not, the wonderful news of a positive pregnancy test is SO EXCITING! But remember the journey is not yet over. A positive test is not necessarily a baby in your arms in 9 months’ time. But it already gives you better than an 80% chance of that much wanted baby. Sadly some IVF pregnancies do miscarry in around 15%-20%  of cases, This is the same as for natural conceptions.  

One of the first questions that I get asked after a positive pregnancy test  is “When am I due?” Everybody wants to know when the baby is likely to be born. For natural conception, obstetricians use a calculation based on Naegle’s rule developed over a century ago.  The rule says that the estimated date of confinement (EDC) is 9 months and 7 days from the start of the last menstrual period (LMP). This was based on checking actual delivery dates and LMPs of thousands of women. This was the average length of the pregnancy. However less than 10% of women actually deliver on that day. 10% of women deliver 3 or more weeks early. 10% deliver more than a week late. So it is just an estimate. You can’t book the hairdresser with any accuracy for the day before the EDC to look your best for the baby photos. Nature is not that predictable.  

For natural conception there is a significant confounding variable in using Naegle’s rule which is related to the variable  day of ovulation. It makes accurate prediction less likely. Obviously the timing of pregnancy requires knowledge of the timing of release of the egg to then be fertilized to produce the pregnancy. Using the Naegle’s  rule there is an assumption that ovulation is occurring  14 days after the start of the LMP. While this true in around 70% of cycles, in the remaining 30% it can vary substantially. For instance , a woman who has a regular 30 day cycle will be ovulating 16 days after the start of her period. So the calculation would always be two days out. More importantly for women whose cycles are varying widely from month to month (say from 27 to 35 days), accurately dating the pregnancy can be impossible based on the LMP since ovulation could be as early as day 13 or as late as day 21. When we are uncertain, ultrasound to measure the length and the developmental stage of the fetus in the first 10 weeks of pregnancy can assist in determining the EDC. 

But IVF pregnancies have a big advantage in assessing the EDC very accurately. In fresh cycles we know when the fertilization occurred to the exact day. Indeed, although not particularly helpful, we know the exact time of the day that fertilization occurred. From that time it will be 38 weeks to the EDC. Some app calculators work on that timing but I find that hard to work out in my head and I don’t have 38 fingers to count on my hands!  We could use Naegle’s rule from the start of the LMP of that cycle but the egg collection and therefore the fertilisation day is variable. We sometimes collect as early as on Day 9 and sometimes as late as day 16 depending on the response of the ovaries.  

So I use the day of transfer as my starting point for calculation of the EDC.  Based upon the standard assumption that the 40 weeks to EDC starts 14 days before ovulation and Day 5 embryos ( blastocysts ) are replaced 5 days after egg collection, that transfer day is 19 days after a “presumed” start of a period. By subtracting 7 days from the 19 (to allow for the +7 days of Naegle’s rule),I take 12 days from the transfer day and then  I can quickly count the nine months on my fingers to tell you your due date.  

The beauty of this calculation is that it can also be used in frozen embryo transfers as well. Often in frozen cycles done with Hormone Replacement treatment using the natural markers is not possible due to  the fact that  there may be no period or the commencement of progesterone to prepare the lining of the womb is delayed due to scheduling of the transfer. The only point of reference is the embryo transfer day. 

So to illustrate with an example or two — 

Woman A started her period on 12/9/21. She started her FSH on 13/9/21. She had her egg collection on 29/9/21. She had her Day 5 blastocyst transfer on 4th October. When is her baby due? 

Based on Naegle’s rule 19th June 2022. 

Based on my more accurate calculation using day of embryo transfer minus 12 days plus nine months = 22nd June. 

Woman B has had a frozen embryo transfer. She had no period because she is menopausal. She started hormone replacement with estrogen tablets and then added progesterone pessaries once her uterine lining was thickened.. Her  transfer of a Day 5 blastocyst was the 19th July 2021. She has had a positive pregnancy test 10 days later. When is she due.  

We cannot use Naegele’s rule cos she has had no period. We cannot use ovulation day as a marker since she did not ovulate. We have to do my calculation. So we subtract 12 days from 19th July and add 9 months = April 7th 2022. 

See it is easy!  But even easier using the Due Date Calculator on my website