Understanding Society User Support

Good afternoon!

I was wondering if you could help me. I am working with Stata and currently have a dataset where children are the main unit of observation combined with their parents' info - pidp, education and so on. All waves are merged, and sorted by child pidp and wave in a long format.
I would like to know the birth weight of the children, which is in newborn files. I created a file with all newborn waves merged together. However, some mothers are mentioned multiple times and therefore it is not allowing me to merge by mother pidp. Is there an easy way to do it?

Kind regards,

Michaela Kyclova