[thaumasiotes]

It is very reliable indeed, but there's still variation.

Compare https://pubmed.ncbi.nlm.nih.gov/3726904/ :

> Data that were collected prospectively from a child health study conducted in Gaza show a strong relationship between breastfeeding and two major components of birth intervals, the postpartum anovulatory period and the waiting time [from resumption of menstruation] to conception.

> The finding of a strong positive association between breastfeeding and the length of postpartum amenorrhea is as expected from numerous other studies.

[fwip]

The effect appears reliably when looking at groups of people. It is not a reliable method of birth control.