I can't speak from any experience on this matter. My baby has never had a drop of formula, and yet my periods came back the month after my pp bleeding stopped (about 10 weeks pp). I breastfed exclusively for 6 months and now that my DD is 16 months old, I still bf twice a day. I am also 6 weeks pregnant with no. 2. We tried to sway blue as you'd know since we bought a plan, but our desired age gap (2 years) was determined before we knew that DD was a girl.
I did see this the other day though and thought it was interesting. It relates to breastfeeding, but talks about how girls tend to be breastfed for shorter durations than boys because they hypothesise that after a girl, mothers are hoping for their fertility to return more quickly so as to be able to try for a son. I was an exception, but from what I've read, breastfeeding typically delays ovulation.
https://www.ncbi.nlm.nih.gov/pubmed/22148132