They're basing it on the date of the woman's last period, not ovulation, meaning that there isn't even an ovum available to be fertilized yet (unless you count the ones still in her ovaries -- and I wouldn't be shocked if they do).
Not that I support this bill -- it's obviously a back-handed attempt at further limiting abortion through technicalities -- but there is actually some medical basis for the way they're calculating the weeks, as pregnancy is counted from the first day of the woman's last period rather than the date of conception. If a doctor says a woman is twenty weeks pregnant, (s)he really means that the expectant mother is estimated to have been carrying the fetus for roughly 18 weeks (including the initial zygote, blastocyst, etc. stages). This is done because it's too difficult to estimate the date of conception and calculate from there. Accordingly, they could actually make a legal case for this bill, based upon the language in the 20 week law (assuming it says 20 weeks pregnant, rather than 20 weeks fetal age).
Like I said: Back-handed bill based on technicalities.
