For anxious spouses, a baby may be a rival. A new child can spark feelings of jealousy in a person who already fears being abandoned by his or her partner, research suggests. A new study found that partners who showed signs of relationship anxiety before the birth of their first child were more likely to be jealous of the child after it was born. Source 1s.
I think in a lot of these relationships if you're truly confident about your own partner's love then it is what it is. Trust comes a long way here and if you actually took the entire path of getting married or having a child with them while still having the pit of doubt or fear in your chest that they might leave you all the time then perhaps you should've thought twice into taking further steps in your own relationship?