They were called mothers of the believers in regards to respect and having high regard for them and this motherhood does not indicate anything that is impermissible or permissible except for respect because indeed it is obligatory upon all Muslims to respect them. As for the impermissibility of marrying them after the Prophet then that is regarding the impermissibility of demeaning the status of the Prophet wherein his wives are not permissible for whosoever is after him forever.
For this reason the legislator has placed four months and ten days (waiting period) for one whose husband passes away as a form of showing respect for the rights of the deceased husband, for indeed that is from the rights of the deceased. And what indicates this is that a woman waits for four months and ten days even if she still has menstrual cycles or barren. And it is not opposing to this that the waiting period of the pregnant women whose husband dies ends with giving birth to the child even if it is less than four months, because we say: when the waiting period passes she is separated from her husband and is free from him and therefore there is no connection that remains for the husband to her. This is why the waiting period ends by the birth of the child.
Shaykh ibnu Uthaymeen – Fataawaa nur alaa ad-darb v.12 p.261