perhaps he is referring to this research (or something similar) https://obgyn.onlinelibrary.wiley.com/doi/full/10.1016/j.ijg... (though this was done it relation to developing countries)

"Conclusion

Considering both the increased risk of mortality and undernutrition for a birth earlier than 36 months and the great number of births that occur with such short intervals, the author recommends that mothers space births at least 36 months. However, the tendency for increased risk of neonatal mortality for births with intervals of 60 or more months leads the author to conclude that the optimal birth interval is between 36 and 59 months. This information can be used by health care providers to counsel women on the benefits of birth spacing. "