Complex sentence: The harsh nature of the German occupation however reduced the birth rate, in Poznań at the end of the war the birth rate was near zero, in Łódż and Innowrocław the birth rate was negative-they were more deaths then births.
Split into simpler sentences: The harsh nature of the German occupation however reduced the birth rate. In Poznań, at the end of the war, the birth rate was near zero; in Łódż and Innowrocław, the birth rate was negative-they were more deaths then births.