It's probably unlikely that there's anything that changes a woman more than becoming a mother. The experience of the bodily changes, the physical growth and the cognitive knowledge that a baby is growing inside, is not comparable to anything else a person will ever experience.
Then the birth of the baby, the courage and strength required, all for the sake of bringing the baby into the world. The sense of responsibility when taking the baby home is incomparable to even the most responsible job. Keeping a child alive is the bare minimum required. Knowing what a baby needs without language to explain. Learning to cope with your own needs as well as be ever present to the needs of the baby takes enormous skill and presence of mind, all when tired and coping with the post-pregnancy body.
It's no wonder that the woman will change with so much to think about and take care of. The emotional connection with the baby can be so intense that the mother might find it hard to connect with others, including a partner. That relationship changes too and ideally the couple can grow together through the parenting experience. The change can be too much for some and the relationship can start to see the early signs of deterioration. The mother can feel torn between the mother and the partner - it's likely the baby will win if a choice is required.
Mothers can experience a lifetime of struggle to balance their mothering role with their own identity, of which mothering is one part of course. Being connected but separated can be a challenge. Being available but having space can seem impossible. Having energy and time for one's own pursuits can be lifesaving and heartening. Getting the balance between mothering, time for self and time for others is an ongoing journey, adapting as children grow.