I knew at 3 1/2 weeks with mine, but my mum didn't know until she was 5mths with me. I was a placenta previa baby and my mum would still get AF every month (or what she thougth was AF but was more than likely breakthrough bleeding from the placenta position), she went to the Dr because she didn't know why she was feeling so wretched. I was born at 8mths gestation so a very quick 3mth pregnancy for my mum, added to the fact she spent the last 3 weeks before I was born in hospital confined to bed due to possible placental abruption.

My mum was 36 when she had me and my brother was 11 and my sister 8, so I certainly was a surprise baby