If you really need an implementation that works in all modern browsers, a simple keyframe animation is the way to go.

So? It’s not like pregnant women are planning weeks in advance for an exact date to give birth. An estimated due date is exactly that: an estimate. I don’t have exact statistics on hand but if I remember correctly, your 10% are even a bit high and it’s more like 3% on the exact date. But about 50% are within +/- one week of the original due date and 80% are within +/- two weeks which is pretty good accuracy for a 40 week time span¹. If you adjust based on ultrasound results, you can get even more accurate estimates but the original due date gives you a good timeline when those ultrasounds (and other examinations) should be done.

¹ seriously, try estimating any other 40 week project to within a week with 50% accuracy.

The latter. This is not about politics but about medicine and it’s nothing new. Calculating weeks of pregnancy from the previous menstruation is generally a lot more reliable in predicting the date of birth than calculating from conception.

The mother may not remember the exact day she had sex, she may have had sex multiple times and not know which time led to conception and as an additional hurdle, sperm may need multiple days to to reach the egg so even if she had sex only once and remembers the exact date, that doesn’t really help to know when the egg was fertilized.

On the other hand, there is a relatively narrow window (a few days) during a cycle when fertilization is the most likely, so calculating from a known point of reference relative to her cycle gives good results.

There is no “new calculation”. This is how medical professionals have calculated weeks of pregnancy for decades if not centuries. The reason is that it’s a lot more reliable than going by date of conception (which is not necessarily the date the parents had sex. Sperm can take days to reach the egg).

Source: I’ve spent 12 years writing accounting and quality management software for midwives.