Is The Innocents really based on "true events"?

The 2016 French film The Innocents is yet another European examination of the aftermath of World War II; as this is such well-worked ground, recent films have sought out - and found - unusual and unexamined niches in this territory - one of which has been the religious outposts, convents and monasteries, and how they endured during such massive trauma and upheaval. Like the recent Ida, The Innocents looks at a Polish convent and develops from this unlikely setting an excellent and though-provoking drama. The plot: A nun or novice leaves the convent and persuades a French nurse assigned to a French military hospital to come to the convent for an emergency; one of the sisters is pregnant and struggling w/ a breach delivery. We soon learn that many of the nuns are pregnant; the convent had been over-run for three days by Russian troops, who repeatedly raped and assaulted the women. Against orders and at great risk, the nurse makes repeated visits to the convent to help the nuns through their medical crisis; but a great moral and religious crisis ensues: What to do about the babies? I won't give anything away. The narrative is well paced, tense, sad, troubling, and to my mind credible. The film states at the outset that it is based on a true incident, but neither the closing sequence nor the credits give any information about the source material, except to say that the movie is "based on a story idea" by so and so. Hm. In any event, another fine film within the European tradition of historical realism.