Passerby anon agrees, however this is fandom, anything is possible.

If OP does want to keep things genuine you could use scent instead. The moment there is a development in your body your scent changes slightly. Enough for a dog or wolf and thus I imagine also for a werewolf to pick up. As such, a werewolf with a strong sence of smell could pick up the subtle change before the woman learns of the pregnancy.