Is it permissible to break the fast in Ramadan and make up the days in order to maintain the weight required for the success of an IVF procedure?

It is not permissible to break the fast in Ramadan for the condition you mentioned, as it does not fall under the categories permitted by Islamic law, such as the sick, the traveler, the pregnant, or the breastfeeding woman. Therefore, your fasting does not cause harmful hardship. As for the effect of fasting on fertility, it is not one of the necessities that permit breaking the fast for several reasons: Firstly, there are 11 months between two Ramadans for treatment. Secondly, one can follow methods of rest and nutrition during fasting to maintain weight and avoid exhaustion. Thirdly, maintaining weight plays a role in fertility, but procreation is in the hands of Allah the Almighty. Allah the Almighty says: (To Allah belongs the dominion of the heavens and the earth; He creates what He wills. He grants to whom He wills female [offspring], and He grants to whom He wills male [offspring]. Or He makes them [both] males and females, and He renders whom He wills barren. Indeed, He is Knowing and Competent.) Ash-Shura/49-50. Therefore, one must follow the means for conception without compromising the obligatory fast and strive in supplication.