Praise be to Allaah.  

It is not permissible to prevent pregnancy except in cases of necessity where competent doctors have stated that giving birth will cause the mother to become exhausted or ill, or will make her sickness worse, or there is the fear that pregnancy or childbirth will most likely cause the mother to die. In that case it is essential to have the husband’s agreement and consent to prevent or end pregnancy. When this reason is no longer a factor, then the woman should go back to not using contraception. 

Fataawa al-Mar’ah al-Muslimah, 2/977 

Because losing weight does not reach the degree of necessity mentioned above, it does not come under the same ruling, especially since this operation to tie the tubes requires the female doctor to uncover and touch the ‘awrah. It is even worse if the doctor is male, so this is another reason not to allow it. 

But you have to follow the guidance of the Prophet (peace and blessings of Allaah be upon him) with regard to food. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The son of Adam does not fill any vessel worse than his stomach. It is sufficient for the son of Adam to eat a few morsels to keep him alive. But if he must (fill it), then one-third for his food, one-third for his drink and one-third for air.” (Narrated by al-Tirmidhi, al-Zuhd, 2303; classed as saheeh in Saheeh Sunan al-Tirmidhi, no. 1939) 

You should also follow the permissible means (of preventing pregnancy), such as withdrawal during intercourse (‘azl). The correct scholarly view is that there is nothing wrong with ‘azl even if there is no reason for it, because of the hadeeth of Jaabir: “We used to practise ‘azl at the time when the Qur’aan was being revealed.” (Narrated by al-Bukhaari, al-Nikaah, 4808).

And Allaah knows best.

 Fataawa al-Mar’ah al-Muslimah, 2/658

 Perhaps what Allaah has decreed for you of children will be better for you than you think, and they will be a blessing that is stored up for you, especially in old age.