“whether after a few children it would be permissible to have a surgical operation on the wife to prevent further conception”

As to your desire and that of your husband to avoid any action which would permanently prevent you from bearing children, the only text we have so far found on the subject is in a letter to an individual believer from the beloved Guardian. The question asked was whether after a few children it would be permissible to have a surgical operation on the wife to prevent further conception. His reply was that such an act was inacceptable and unworthy, and those who commit the act were responsible before God.

When the Guardian was asked whether the exercise of birth control constitutes a sin in a case where the number of children would prevent the father from fulfilling his obligation to educate his children, he stated that it is the duty of Baha'is to uphold moderation in all things, and avoid illegal methods.  

- The Universal House of Justice (From a letter to an individual believer, July 31, 1970; compilation: ‘Lights of Guidance’) 

November 1964: The Universal House of Justice arrives at two decisions concerning the Hands of the Cause

“after study of the sacred texts and hearing the views of the Hands of the Cause themselves, has arrived at the following decisions:

There is no way to appoint, or to legislate to make it possible to appoint, Hands of the Cause of God.

Responsibility for decisions on matters of general policy affecting the Institution of the Hands of the Cause, which was formerly exercised by the beloved Guardian, now devolves upon the Universal House of Justice as the supreme and central institution of the Faith to which all must turn. 

- The Universal House of Justice (‘Messages from the Universal House of Justice, 1963 to 1986’)