She needs to keep her weight steady because she wants to have a baby, but she loses weight because of fasting. Can she break the fast in Ramadan?
Praise be to Allah
Islam allows breaking of the fast during Ramadan for those who are sick or travelling, and for those who have reached such a great age that they are unable to fast, and pregnant and breastfeeding women if they fear for their babies or themselves. It allows these people to break the fast because of difficulty or fear of harm or death.
The situation that you have asked about does not come under any of these categories for whom it is permissible to break the fast, and cannot be compared by analogy to any of them. Fasting does not cause you difficulty that will lead to harm. Please see the answer to question no. 38532.
With regard to its impact on having children, as you mentioned in the question, this does not come under the heading of necessities that make it permissible to break the fast, for the following reasons:
You have eleven months between one Ramadan and the next, which is sufficient time to focus on treatment and trying to resolve the matter.
Whilst fasting, you can follow some methods of rest and nutrition that will help you to keep your weight steady, and avoid physical exertion. You can also restrict yourself whilst fasting to obligatory duties only, and get a lot of rest.
Keeping your weight steady, as the doctors say, plays a role in fertility, but keeping it steady does not guarantee that you will have a child. The matter is entirely in the hand of Allah, may He be exalted.
Allah, may He be exalted, says (interpretation of the meaning):
“To Allah belongs the kingdom of the heavens and the earth. He creates what He wills. He bestows female (offspring) upon whom He wills, and bestows male (offspring) upon whom He wills.
Or He bestows both males and females, and He renders barren whom He wills. Verily, He is the All-Knower and is Able to do all things”
Based on that, you can take some measures to help you get pregnant and have a baby without giving up on the obligatory fast. You have to strive hard in supplication, then the matter is entirely in the hand of Allah, may He be exalted.
And Allah knows best.