A heavily pregnant woman was attacked by a teenager who demanded she hand over her mobile phone.
The victim, who is 35 weeks pregnant, was pushing her three-year-old son in his buggy through the alleyway from Station Road to Carnegie Street in Rushden when a boy approached her and demanded her phone.
When she refused, he grabbed the buggy so she moved between him and her son.
The offender then pushed the woman and she hit her head against the wall.
He then made off towards Carnegie Street.
A police spokesman said the victim suffered a large bump to the head and a graze to her hand during the assault which happened between midday and 12.20pm on Wednesday, January 21.
The offender is described as 16 or 17-years-old, about 5ft 10in, slim build and wore dark clothing.
His clothes appeared dirty and he smelt of cigarettes.
Witnesses or anyone with information is asked to contact Northamptonshire Police on 101 or information can be given anonymously to Crimestoppers on 0800 555111.