The woman was shoved off the 173 bus at a stop on Ripple Road, in Barking.
A spokesperson for Metropolitan Police said the victim was hit repeatedly and had her mobile phone smashed when she tried to call for help on Monday.
A 28-year-old man has been arrested on suspicion of actual bodily harm, assault on police and resisting arrest.
The 30-year-old woman had been travelling with her nine-month-old baby and five-year-old daughter when she was attacked.
She was struck in the face, chest and shoulders and was taken to hospital for treatment to bruising to her face and upper body.
Neither her children or unborn baby were harmed in the attack, which police described as "despicable".
Det Sgt Beverley Williams said the woman's daughter had been "left traumatised after seeing her mother being assaulted".
"I am appealing to anyone who was at the bus stop or on the bus who has yet to come forward to please get in contact and assist us with our investigation," she added.
A pregnant mother was pushed off a bus and punched by a man who said there was not enough room on it for a pushchair with her baby in it.