Is there a certain point where alchohol starts affecting pregnancy?

Okay so this might be a weird one, i was watching a series and one of the characters hadnt had their period and was worried they could be pregnant. Her friends brought her to a bar to talk about it and they all got wasted. This got me thinking, how come there will be times where women will be pregnant (without knowing) and still drink but it wont harm the baby? is there a certain age it can mess them up at??