Adam Williams and Rachel Bradley start the series about a year and six or so weeks into their relationship. Jenny and Pete Gifford are a married couple that start the series with the birth of their first child. Karen and David Marsden are another married couple, they already have a child, and are thinking of having another.
The three couples end up friends after going through the 'friends of friends' process. Jenny, Pete and Adam are friends, and Adam's relationship with Rachel pulls Karen (Rachel's friend) and David (Karen's husband) into the friends relationship.