Yes, anyone can post anywhere.
But, if you are going to post in a topic, then it does make sense to use the appropriate one. Like if you want to talk about yoga, you use the yoga topic not the cycling one.
If you are TTC or trying to establish if you have conceived, use 'conception'
If you want to ask about a pregnancy, use pregnancy.
If you want to chat about being pregnant, use antenatal clubs.
If you want to talk about car seats, pushchairs, nappies etc, use their product topics or parenting.
There really has been an outbreak of posts not about pregnancy in 'pregnancy' in the last couple of weeks.
And I find it really odd, as there are shortcuts to both conception and antenatal clubs at the bottom of the Active Convos page, so it's not even as if it's the easier topic to find.