Stretch or dilation of the cervix and vagina are strong stimuli for oxytocin secretion, mediated by neural pathways called the Ferguson reflex.

Article: https://www.sciencedirect.com/topics/neuroscience/ferguson-reflex

"Upon application of pressure to the internal end of the cervix, oxytocin is released (therefore increase in contractile proteins), which stimulates uterine contractions, which in turn increases pressure on the cervix (thereby increasing oxytocin release, etc.), until the baby is delivered.

Sensory information regarding mechanical stretch of the cervix is carried in a sensory neuron, which synapses in the dorsal horn before ascending to the brain in the anterolateral columns (ipsilateral and contralateral routes). Via the median forebrain bundle, the efferent reaches the PVN and SON of the hypothalamus. The posterior pituitary releases oxytocin due to increased firing in the hypothalamo-hypophyseal tract. Oxytocin acts on the myometrium, on receptors which have been upregulated by a functional increase of the estrogen-progesterone ratio. This functional ratio change is mediated by a decrease in myometrial sensitivity to progesterone, due to an increase in progesterone receptor A, and a concurrent increase in myometrial sensitivity to estrogen, due to an increase in estrogen receptor α. This causes myometrial contraction and further positive feedback on the reflex.[1]"

https://en.wikipedia.org/wiki/Ferguson_reflex