If your child is old enough to really understand pregnancy or that a sibling is on the way, when did you tell your child that you were pregnant?

DS1 was only 2 when I was in the early stages of pregnancy with DS2. I think we told him pretty early because he would be rough with me and we didn't want him to be so rough. But now DS1 is almost 4 and knows perfectly well what a new baby means. So I'm hesitant to tell him anything about the pregnancy until later. I'm 9 weeks and thinking that we'll tell him if our NIPT test results come back ok. So probably around 11-12 weeks. DS2 is only 15 months and he probably won't understand any of it until the baby is here