Bismihi ta'ala

This is something that the couple should have outlined prior to their marriage, especially if the husband or the wife does not have any interest in having children. After marriage, although the husband or his mother cannot "force" the wife to have children, she should see this from a different perspective. 

Our religion encourages us to have children, and many of them. There are numerous narrations from Ahlul Bayt (a.s.) that stress on having multiple children, and praises a woman who bears children. Motherhood is the most honourable thing a person can achieve, so what would want a wife to avoid having such a status.

She does not need to be forced. She only needs to look within and realise that it is going to be the greatest thing in her life. 

It will also keep her marriage and strengthen the bond between her and her husband. 

If, for example, she is having trouble with her husband, and she does not know whether her marriage will last or not, so she thinks pregnancy will be a bad choice. Or there is violence in the household, and she does not want to have a baby to be in such an environment. Or other situations that would jeapordise her relationship with her husband, then she must be very frank and straightforward about this. 

Most important for her to know is building a family is the most valuable thing a person can do. 

With prayers for your success.