I live somewhere maternity leave is shorter and it is quite normal for babies to go to daycare or a childminder 5 days a week from 3 or 4 months old. Being a SAHM is not common unless you are a foreigner (in which case not working probably has a lot to do with not speaking the local language or having transferrable qualifications).
I initially felt very unhappy about the prospect of a short maternity leave (and I still think 4 months is too short and luckily I was able to extend it to 7 with holiday and some unpaid leave).
The babies are not traumatised and neither are the parents. Childminders actually tend to say the settling in period is much easier when the baby is only 3 months old because after 6 months they start to get separation anxiety. My son started with her when he was 8 months old and within a few days it was all fine.
At the end of the day, your baby won't remember whether you took a long maternity leave or not. My mum went back to work part time when I was 5 and my brother was 3, and I have very few memories of before that age. I have a couple of vague memories of nursery school, and I remember one day when I was in reception my mum forgot to pick me up from school because she was busy doing a puzzle with my brother at home. I have much clearer memories of being 8 or 9 years old and always being the last to be picked up from school because my mum (who was a teacher) thought she had time to finish her job at a school on the other side of town and come and pick us up (even if she had jumped in her car within 30 seconds of the bell going it would have been tricky, and she is a faffer).
The point is, children aren't traumatised by going to nursery school or a childminder, or going to after school clubs or being picked up from school by a childminder. Long maternity leaves and staying at home with the kids are, I think, more for the mother than they are for the child, who will be fine either way.