At 7.5 to 8 pounds and about 21 inches long, your baby is physically ready to be born now, but for some reason she feels like staying put for a while longer. There's really no more moving-around room in there, so she can't be all that comfy, but she's not feeling the need to make her appearance just yet. What she is feeling the need to do this week is produce stress hormones -- lots of them.
As you read this, your baby's endocrine system is gearing up for childbirth. No one's totally sure what triggers the birth process, but there's a good chance it has something to do with a hormonal signal from the baby to the placenta. As you can imagine, birth is a pretty stressful experience for a baby -- in fact, she'll be making more stress hormones then than she will in her entire life.
As you produce hormones during labor to help you deal with the pain, it will trigger the baby's adrenal glands to make catecholamines, or neurotransmitter hormones (think adrenaline and dopamine). These natural chemicals will help her breathe and increase her blood flow, energy supply and alertness. So, it's possible that her increased time in utero will allow her to be better adapted to life on the outside.