A heavily pregnant woman and another female were both allegedly assaulted by a man swimming in the River Witham in Boston.
The woman, who was nine months pregnant at the time of the incident, was sat under the railway bridge on Witham Bank East with a man and another female on Saturday, June 30.
The man who had been swimming in the river then approached the group and assaulted both of the women at around 7.30pm that evening.
A second man, who had also been in the river, later approached the group carrying a knife. The group then left and phoned the police.
A 32-year-old local man was arrested on suspicion of assault and has since been released under investigation.
Chief Inspector at Lincolnshire Police Deborah Clark said: “This was an attack on a heavily pregnant woman and the consequences could have been devastating.
“We are appealing for witnesses who may have seen the assault to contact us and to assist in our investigation.”
Anyone who witnessed the assaults or has further information should contact police on 101 or via email at [email protected] quoting incident number 449 of June 30.
It can also be reported through the independent charity Crimestoppers anonymously on 0800 555 111 or online here.