Not engaged by 38 weeks

At my 38 week appointment I was still 1cm dilated & 50% effaced. I asked the dr if the head was engaged and he said that it was "appropriately low" but not engaged. Now I wish I had asked more questions because I'm stressing out that either my pelvic structure is not big enough or that baby is in the posterior position! Anyone else having a similar issue or have any understanding of this? I've read that first time moms are usually engaged by 38 weeks.