I'd have to disagree. For an average woman who has regular periods, sure, she'd probably notice. But 1) not all women track their periods or pay that close attention and 2) not all women have regular periods, and 3) not all women are well educated about their periods. It's not implausible at all for a woman that early on to not know she's pregnant. Trust me, I work in an emergency department and we see plenty of women coming in who find out they're pregnant on routine testing. They're usually about 6-7 weeks along.