Your relationship with your mother-in-law is naturally different from your relationship with your own mother. The birth of your child and her grandchild could bring you closer together, or on the other hand it could lead to a difficult relationship. In this case, you need to sort any problems out before they get any worse. Use your prerogative as a young mum, or else trust your own mum to help you ease relations between you and your mother-in-law.
Getting her to back off
If your partner is very close to his mum and daren't contradict her, don't count on him to help calm any conflict over choosing names, clothes, feeding, and anything else you may disagree on. Make things clear from the start: of course you understand that she wants to be a part of things and is excited about the birth, but at the same time she has to understand that it's your baby, not hers.
Seeing your mum in a new light
Even if you thought your mum wouldn't take too kindly to the birth of your child, you could well find she'll go completely goo-goo as soon as your baby is born. Many women who have had too-distant relationships with their own mothers find they rediscover their own mum and see a kindness and generosity they didn't see before after the birth of their child.
A female bond
After giving birth, you can understand what your mum went through when she gave birth to you. Creating a lineage creates a maternal bond between you. You'll realize what your mother gave you when she gave you life, but of course you'll bring your own child up differently. You know you can count on her during difficult times over the years to come, which will see so many changes in your life.