Thats why birthing with a posterior baby is often longer and pushing is harder to get them out.
When a lady has birthed vaginally before the birth passage opens easier because its being used before and the cervix stretches more effectively and most of the time a woman birthing for the 2nd, 3rd etc time will not have much difficulty when the baby is posterior.
my second was born posterior and it took me twice as long to push her out. i didnt think she was ever going to budge