Mmm, this really is a fascinating area of science...
The reality is that "38 weeks from fertilisation" was only ever a guess at how long a normal human pregnancy lasts.
There is now some evidence that "28 weeks from when embryo hits X size" might actually be a better estimate - so its not that anyone doubts the date of conception - we all know that with IVF anyway - its that the date of conception may not actually be the best predictor!
There are lots of examples of this in the animal kingdom where development takes variable lengths of time in the early stages of fetal development and then becomes more consistent later on. pandas for example can be pregnant for anything between 95 and 160 days! Crocodiles also have very variable gestation.
So as Crazy Horse says - IVF is actually resulting in a lot more knowledge for science on this!