Wife has small pea sized lump left breast, it's located on the inner side near the sternum, not quite in breast tissue, to me it feels like muscle knot. shes pregnant. mamo next week, worry?

Not yet - need more: Information. Make sure to get a complete workup, starting with the mammogram, and probably also getting an ultrasound of the lump. It may require a biopsy or removal of the lump to look at it under a microscope if imaging cannot definitely confirm a benign finding (for example, a simple cyst). However, given your description, it should be easy to remove the lump in a simple procedure.

Breast lump: If the lump is hard, the first step could be an ultrasound to determine if the lump is a cyst or a solid mass. If she is 40 weeks pregnant, waiting 1-2 more weeks until after the delivery is an appropriate course of action before doing a mammogram. It is most likely an enlarged milk duct or lymph node, but it is important to follow through.