Between 5.30pm and 6pm she contacted her partner asking how far away he was from returning from work.
Sometime between the time she contacted him and the time he returned home, she had injured her son and called emergency services for help.
Judge Arthur Tompkins said the woman's memories of what happened in that time became "somewhat blurred" according to a psychologist's report.
"The precise way in which the injuries, subsequently revealed by medical exam, occurred is not entirely clear," he said.
"It's clear whatever happened, her assault caused the young victim a serious head injury together with other injuries to his ribs."
The baby has since recovered and there are no long-term effects from his injuries.
The baby was uplifted by Oranga Tamariki after the incident but after working with family, the courts and professionals, the woman has since regained custody of him.
Her lawyer Lucie Scott said based on multiple reports, the court could be assured an incident like this would never happen again.
Scott said Oranga Tamariki had confirmed it would not be intervening, and agreed the woman is a loving and attentive mother who has taken steps to make positive changes in her life.
She added that a report made by a psychologist for the court talked about repressed memories and the impact trauma has on what one person can recall as the truth.
Judge Tompkins said trauma perhaps came from seeing her son hospitalised and when he was later removed from her care by Oranga Tamariki.
A psychologist who spoke with the woman said she described her life at the time to be like "trying to be Wonder Woman", balancing household duties, getting used to motherhood and getting her son into routine, and her own mental health.
Judge Tompkins said work she had done since that day showed she was much more equipped to deal with stress and sentenced her to nine months' supervision.
