Complex sentence: Higher levels of oxytocin help to contract the mother's uterus and expel the placenta, and this increase in the hormone oxytocin also shows a lower risk of developing postpartum depression.
Split into simpler sentences: Higher levels of oxytocin help to contract the mother's uterus and expel the placenta. This increase in the hormone oxytocin also shows a lower risk of developing postpartum depression.