[skissane]

In practice, you normally don't know the exact date of conception, you can only roughly estimate, but any estimate is likely to be a few days out. There are exceptions – e.g. IVF, or the parents know they only did it once and remember the exact date they did it on (but their memories may be fallible, etc). Birth is better because (in developed countries where babies are normally born in hospital) the bureaucracy almost always knows the exact date every person was born – in many cases even the time of day, not that anyone really cares about that.

Plus, any proposal like that is inevitably going to get mixed up with the politics of abortion, which is another reason why it won't happen.

[wzdd]

Conception as a result of sex can take place up to one week after the act, even if it only happened once.

[Qem]

A error margin of +/- 7 days still beats a error of 9 months.