Is it permissible to become pregnant immediately after childbirth, out of fear of being unable to breastfeed the first child who is growing with difficulty?

Islam encourages having many offspring, so there is no harm in becoming pregnant immediately after childbirth, unless a reliable doctor determines that pregnancy poses a danger to the infant's life, in which case it must be delayed, based on the Prophet's (PBUH) saying: "There shall be no harm nor reciprocating harm."