Well, we breastfed.
Problem is that isn't actually a reliable solution since not all mothers produce milk at the quantities their child needs. This is particular true as a child grows, many mothers grow tired of breastfeeding and look to formula as an alternative. Problem is this deteriorates their milk production since the child is eating less breastmilk.
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The reality is this situation is primarily dangerous for mother's who cannot breastfeed or lack sufficient production - and require formula. There is no go, safe, reliable option because Option A (Breastmilk) is gone and Option B (Formula) is as well.
People keep acting like there _should_ be a backup to formula, without understanding that humans are the only animals on earth that have created an alternative to their naturally produced milk.