If you make the statement, "My life will not change when I have a baby" to a parent, you will most probably be met with a look of disbelief, or a smirk, or perhaps even an outright laugh! (Famous last words!) New babies place a whole new perspective on the lives of couples. A third person (or a fourth or more) coming into the dynamics of a one-on-one relationship, mean that each parent not only has to consider the different needs of each other (which will usually change dramatically after the birth) but also the needs of their new son or daughter.
For some couples, the conscious decision to start a family will bring with it feelings of contentment and a sense of 'settling down'. For others, it may bring a sense of commitment to the relationship, and an endorsement (or 'consummation') of their relationship on another level. Parents considering a subsequent child may need to think about how many children they want. Each partner may have their own expectations about how many children they would like (or can afford).
Being able to cope with changes in your lifestyle after the baby arrives (even if it is for the better) is important. Newborn babies are very demanding and caring for them in the first few months can be particularly tiring. If you have mismatched expectations about your roles in parenting and who does what in regards to household chores (and perhaps different approaches to parenting), this can place incredible demands on even the most stable of relationships. If your relationship is not strong, then deciding to have a baby in the hope of solving your relationship problems will usually only magnify existing difficulties, as well as introduce new ones. While babies can enrich a relationship, they can also be the reason for a relationship breakdown.