Even with the best prenatal care, sometimes your baby wants to arrive a little earlier than planned. If this happens, your hospital will want to make sure your baby can travel safely in the car. Usually, infants born earlier than 37 weeks will need a tolerance test, also known as a car seat test. Babies with other medical conditions or those who may not tolerate sitting in a semi-reclined position, such as a rear-facing car seat, may also need this test.