If you miss a fast due to illness, travel, or pregnancy, you must make it up [qaḍā] at your earliest convenience. Delaying it unnecessarily is discouraged. If you missed more than one you may either make those up consecutively or intermittently, but consecutively is better.
Only if you are unable to fast due to a chronic illness or old age will you not make up fasting since the situation is expected to persist for the rest of your life. In this case, you must feed a poor person, two average meals, for every day of fasting missed.
Shaykh Mustafa Umar