With my first and third I knew almost immediately as we were actively TTC and used clomid with DS1 and alternative therapies with DS3.

DS2 was an oopsie, and I had no idea I was pg until nearly 7 weeks. We conceived without AF returning after I'd had DS1 (there's 20 months between them).
I only tested with DS2 because I'd had several days of feeling midly nauseous and I wanted to go the chemist to get something for it. I knew I'd be asked if I was pregnant and I remember joking with DH that I'd better do a test just so I could honestly tell the pharmacist I wasn't. The joke was on us!!