A woman “let go” of a pushchair, causing it to roll into the path of a lorry on the A16 in Lincolnshire.
A “bundle” was seen to “fly up into the air”, but no baby was found during “extensive searches”, a spokesperson for Lincolnshire Police said.
The incident, described as “very distressing”, happened close to the junction with Middlegate Road West, in Kirton, at about 12:20 GMT on Monday.
Officers spoke to the driver and were keen to trace the woman, the spokesperson added. She was described as being in her 20s and wearing a three-quarter length jacket and trousers.