My breastfed baby took to the nipple immediately, no issues. He’s 6 months old & the slow flow still works just fine for him. High quality, beautiful color. I bought the blue cap set to fit onto my natursutten bottles. The screw on lid part DOES NOT FIT on natursutten glass bottles. BUT the nipple fits perfectly & the set came with a matching funnel. That makes my almost $6 purchase well worth it. Natursutten Question: I bought the bibs set to fit onto my natursutten bottles bc those bottles don’t have a vent in the nipple. The vent is actually another plastic piece. I wanted to find another compatible nipple with a vent already in place so that I have less pieces to wash & zero plastic in my babies bottle. So happy that these fit, they are sturdier & more affordable! I might just buy a bibs bottle to use the lid part bc the color is so pretty! Final opinion, I highly recommend Avent, natursutten, hevea & bibs bottles. Avent nipple shape may not work for every baby but doesn’t need to be replaced as often as rubber, natursutten has a plastic vent & bibs is the most expensive at first but nipple replacement is most affordable of the rubber nipples. Pick your poison.