I know exactly what DT75 is talking about with regards to her mum. Regret is a harsh word that conjours up terrible images, but I can think of 6 parents off the top of my head that have three kids but wish they only had two. It doesn't mean they love that child any less, or treat them any differently, and it doesn't mean they have a mental illness. It simply means that they ended up beyond their limits of the amount of children they 'should' have raised and are able to recognise that about themselves.
I do wonder with these threads if people forget that parenting is still hard beyond the baby years. All that is ever discussed is getting through the first few weeks/months.