It is not permissible for one of the spouses to prevent pregnancy without the consent of the other
Question

                A woman came to a gynaecologist to ask her for contraceptive medicine without her husband’s permission, on the grounds that this woman’s husband had another wife and he had children from her, and she was still studying in the university. Is it permissible for the doctor to prescribe this medicine for her or should she refuse?.
Praise be to Allah.
    It is haram for her to use contraceptives without her 
    husband’s consent, because it is the right of both husband and wife to have 
    children. Hence the scholars said that it is haram for the husband to 
    withdraw from his wife without his wife’s consent. 

    “Withdraw” here refers to ejaculating outside the vagina 
    (‘azl) lest the woman become pregnant. But if both spouses agree to taking 
    these pills, it is permissible, because that is akin to ‘azl, which was 
    practised by the Sahabah, as Jabir (may Allaah be pleased with him) said: 
    We used to practise ‘azl at the time when the Quran was being revealed. 

    In other words, if it had been forbidden the Quran would 
    have forbidden it. However, it is better not to take these pills because 
    that goes against the wishes of the Prophet  (peace and blessings of 
    Allah be upon him) for this ummah, that they should have many children. 

    I tell you that the ones who first invented these pills were 
    the Jews and other enemies of the Muslims, who want to eradicate this ummah 
    and reduce its numbers, and leave it dependent upon others, because the 
    smaller their numbers the smaller their productivity, and the greater their 
    numbers the greater their productivity – in agriculture, industry, commerce 
    and in every field. Nations are feared more than others if they are greater 
    in number, even if they are not advanced in industry, because large numbers 
    alarm the enemy.  

    So we call upon the Muslims to have many children, so long as 
    there are no circumstances such as sickness or poor health on the woman’s 
    part or she cannot give birth without surgical intervention, because these 
    are cases of necessity and cases of necessity are subject to different 
    rulings. 

    Ash-Shaykh Ibn ‘Uthaymeen, Fatawa al-Mar’ah al-Muslimah, 
    2/556 

    Please see also question no. 
    21169. 
If taking these pills is 
    haram, then it is haram to help her to obtain them, because Allah says 
    (interpretation of the meaning):

     “Help you one another in Al‑Birr 
    and At‑Taqwa (virtue, righteousness and piety); but do not help one another 
    in sin and transgression”

    [al-Maidah 5:2].