Ruling on tying the tubes unnecessarily
Question

                I am a 36 year old woman with 6 kids and I am now with my 7th child and would like to know if it is permissible in Islam to tie my tubes when this baby is born?  I don't want to cut and burn them, I want to place a band around the tubes to avoid the egg from dropping all the way down.  I know I cannot stop the will of Allah, I just want to try to loose weight. I weigh more that 250 lbs.  Every time I try to loose weight, I end up pregnant.
Praise be to Allah.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. 

    Fatawa 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 Allah be upon him) with regard to food. The 
    Messenger of Allah  (peace and blessings of Allah 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 Jabir: “We used to practise ‘azl 
    at the time when the Quran was being revealed.” (Narrated by al-Bukhari,
    al-Nikah, 4808).

    And Allah knows best.

     Fatawa al-Mar’ah al-Muslimah, 2/658

     Perhaps what Allah 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.