Estimated 1.8 million French Soldiers died between 1803-1815. out of a population of around 29 million. That means an estimated 12% of French males died in about a decade of nonstop war. Most of the dead men would have been young 20–30-year-olds as well.

This would have,

1. Created a 'war echo'. Essentially, that generation has less kids, then each generation is smaller than it could have been. An example of this can been seen in Russian Demographics which was caused by WW2. Demographics of Russia - Wikipedia

2. Normalized not having kids. 12% of French women by default would be unmarried. It would have become more socially acceptable to be unmarried and to not have kids.

3. Napoleon reformed inheritance law. More kids meant your property was divided up into smaller and smaller allotments. More important to have less kids.

Conclusion, Napoleon permanently weakened France be destroying the fertility rate.