Good article today on the problems of carrying baby car seats following birth. The problem is lifting something when your pelvic floor muscles and core muscles have not recovered their full strength. Lifting and carrying your baby is fine, even you have been diagnosed with a pelvic organ prolapse soon after giving birth (I think 1 in 12 women suffer from POP after birth). The problem is carrying heavier objects and in non-ergonomic positions as the article explains.
You can find advice on how to reduce intra-abdominal pressure (lifting is one contributor) in this leaflet by the Pelvic Obstetric and Gynaecological Physiotherapy.