The human rights group said one witness to the assault on Baga, on the shores of Lake Chad in northeast Nigeria, told them the woman was shot by indiscriminate fire that also cut down small children.
"Half of the baby boy (was) out and she died like this," the unnamed witness was quoted as saying.
Amnesty said this week that hundreds of people, if not more, may have been killed in the attack, which began on January 3 and is thought to have targeted civilian vigilantes helping the military.
"They killed so many people. I saw maybe around 100 killed at that time in Baga. I ran to the bush. As we were running, they were shooting and killing," a man in his fifties was quoted as saying.
Another woman added: "I don't know how many but there were bodies everywhere we looked."
The testimony chimes with claims from local officials that huge numbers were killed and that of witnesses spoken to by AFP, who described seeing decomposing bodies littering the streets.
One man who escaped from Baga after hiding for three days said he was "stepping on bodies" for five kilometres as he fled through the bush.
Nigeria's military, which often downplays death tolls, said this week that 150 people died, dismissing as "sensational" claims that 2,000 may have lost their lives.
Security analysts have said that it may be impossible to know exactly how many were killed, with the town and surrounding area still in rebel control and access impossible.