Personally @Playingintheshadow, yes, I do regret my sibling being born - their impact on my life was overwhelmingly negative. We were so skint, so tired (physically and emotionally) and I was pushed out of the family at a young age as I was 'big enough to not need any attention'
I know DM regretted having them many, many times when they were smaller and extremely hard work - now it's a different story, they are very close, much more than DM & I.
I'm completely pro choice but I find this post quite distasteful:
Having a baby after a gap is less risky now in some respects. There are advanced medical checks that give more certainty that the child will be born healthy.
A well child will have less impact on the lives of your older children and they all should grow up to be happy, independent and friends.
It just sounds like the PP is suggesting that if all isn't well with the pre natal checks, the OP should get rid and start again - and 'they all should grow up to be happy, independent and friends' is just ridiculously optimistic! There are plenty of people who this doesn't happen to. Plus pre natal checks won't pick up conditions like ASD.
There are quite a few PP saying oh, the older children will LOVE a younger sibling, they will have a great time babysitting! And no matter how much the OP will say they won't ask the older siblings to babysit, it will inevitably happen - "just keep an eye on them for 10 minutes whilst I pop to the shops" etc. My personal experience of life with a toddler whilst I was a teenager was general tedium - everything had to accommodate their needs/ wants - plus trying to study in a small house and keep everything away from small, grabby fingers was a nightmare.
Ultimately I think the OP simply has a decision between heart and head - there's no real reason to have another baby other than 'DP and I want one' - she has two happy, healthy children and it depends whether she's willing to gamble their happiness on adding a much younger sibling to the mix. Personally I wouldn't (and haven't), but there you go.