When making the decision to fast, your health and your baby’s health is important, so you must ensure this is not compromised in any way.
Fasting is not recommended if you have a baby under 6 months and you are exclusively breastfeeding. This is especially important in the first 6 weeks or so when your breast milk supply is being established.
If your baby is over 6 months old and you are well then you can fast, but you should take into account how you feel when making the decision.
In Islam, health comes first, so if it is more harmful to fast for any reason then you would be exempt, for example if you have a condition such as gestational diabetes. There are also alternatives, such as fasting at a different time, fasting only on some days, or giving money to charity (fidyah).
So, for example, in a year when Ramadan is in the middle of summer, you may choose to fast in December instead. Or if you are pregnant or breastfeeding and otherwise well, you might decide to try fasting during weekends only.
See this mum’s blog of her experience of breastfeeding 4 children through Ramadan and the decisions she made each time.