the Scientific Research Committee - IslamToday.net
This condition is valid in an employment contract, since it is for the sake of the work being carried out. For many occupations, pregnancy seriously and detrimentally affects work performance.
However, the woman may not accept this condition unless her husband does as well. Having children is a shared right for both the husband and wife. It is permissible, however, for both parties to elect not to have children for a certain period of time.
And Allah knows best.
Source: Islam Today