Make it easy on yourself! Two parties in one month is a lot. Maybe just invite grandparents and a few neighbors over for the baby and don’t go “all out.” Presumably you would invite other children for your 3-year-old’s party, and that requires a lot more work and cleverness so I’d concentrate on that one.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.