Honestly I'd have to say having our daughter. While it has been the most amazing experience for both of us to be parents, it has been a stress on our relationship. We were together for 7 years, just the two of us, before she came along so it was hard to adapt to being 3 after so long of being only 2. I have had physical problems since delivery that have affected our sex life in a not so great way, we've had job changes that have added to the stress - and we will ALWAYS be parents so this is something we will always have to work on. We are lucky, she's been an amazing little girl from the moment she came out. She herself has never been a source of stress, but just the changes that come along with parenting.
We are still over coming these new obstacles. The nature of children is constant change, so we are constantly having to change and adapt with her and ourselves. Things are much better than even 6 months ago - 6 months ago I didn't want to be touched and I wasn't even sure I still wanted to be in a relationship. But we've worked on it and we are getting there. We just have to remember there is still US somewhere in there, as well as parents.