The victim, aged 32, was walking with her two young children when she was attacked between 7.30pm and 7.45pm last night (Sunday).
She was between Finland Way and Copenhagen Road when she noticed a male riding towards her on a red and black bicycle.
As he rode past he kicked the woman in the stomach without any warning.
A police spokesman said: "He is described as being white, of a large build and had facial hair.
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"He was wearing a grey tracksuit and dark cap."
Anyone with information or who may have witnessed this incident is asked to call Northamptonshire Police on 101 quoting reference number 19000695488.