French Birthrates in the 19th Century

it's one of the most puzzling issue of demographics. No one has a really good answer for it.

I have once heard that there was a strong correlation between the collapse of the Catholic church's influence at the end of the 18th cent and the sluggish birthrate. Where the church was still strong (Britany) birthrates remained high.

Another detail is that there was so to speak no emigration from France in the 19th century unlike nearly every single other European country. Thus it may have been a simple strategic decision that favoured lower birthrates over emigration. This is typically the sort of decision that is difficult to account for as it is mostly cultural (in the same way why do Germans save money while English live in debt?).