When I just read your title and hadn’t seen your age I thought to myself ‘anything over 45, probably’.
Loving parents having another baby isn’t itself problematic, and I don’t think there is necessarily a huge difference between 45 and 48. But there are lots of considerations you’ll want to think about.
When your youngest is 18 you will be 66 / 67. Do you feel comfortable offering the high level of support teenagers need in your late sixties? My mum is currently 60 and while she’s very fit and active, I can’t imagine her having the inclination or energy to full-time parent a teenager. She’s also quite far removed from the ‘culture’ of youth now - she would find a teenager hard to relate to.
From a financial perspective - if your children go to university, will you be able to financially support them from your pension? Will you have to carry on working? Are you happy to do that?
It’s very unlikely you would conceive naturally at 48 - have you factored in the cost of IVF?
Could you care for a disabled child? Your risk of disabilities is much higher. Have you thought about the impact this might have on your existing children? How would a profoundly disabled teen / young adult be cared for when you are in your 60s / 70s.
Could you cope if it was twins?
Ultimately, babies bring joy and love. But in your case there are also a lot of risks, and you have two existing kids to think about. Personally, I wouldn’t do it. It would be too great a risk for me, and my priority would be my existing children.