PSA: Why You Should Never Use Medela Bottles to Feed Your Baby

I learned the hard way…

This is a special post for you brainy business babes who are (or will be) moms!

I’ve been nursing my daughter, Baby P, for nearly nine months. That entire time, I used Medela bottles to feed her and store milk. This morning, on a whim, I decided to try sending her milk to daycare in an Avent bottle I bought several months ago, but had never used.

I was surprised when I poured 4 ounces of milk from the Medela bottle into the Avent bottle – it fell way short of the 4-ounce mark. I topped up the Avent bottle and carried on with my day… but I couldn’t get this nagging thought out of my head: which bottle was correct?

Being a bit of a science nerd, I decided to go all fourth-grade-science-fair and do an experiment. Here are the two bottles, you can see that each of them is filled exactly to the 4-ounce mark (using US ounces).

Medela bottle

Medela bottle

Avent bottle

Avent bottle

(FYI – the liquid in the bottles is water with a bit of blue food coloring to make it easy to see. If my breastmilk was actually bright blue, this would be a whole different post.)

I rummaged through my bin of random baby stuff I got for free when Baby P was born, and found these two unopened Gerber disposable nursers, which hold 60 mL (2 ounces) apiece. Perfect for my impromptu science experiment!

Gerber disposable nursers

Here are the results:

Medela bottle results  Avent bottle results

You can see that the Avent bottle measures exactly 4 ounces, but the Medela version of 4 ounces is actually 5 mL short.

That may not seem like a big deal, but if you are giving your baby six 4-ounce bottles a day and losing 5 mL each time, you are feeding that little one a full ounce less every day than you thought you were!

And that’s if you are being an absolute stickler for measuring exactly to the 4-ounce line. I confess that, as a sleep-deprived new mom, I mostly eyeballed it.

This explains some strange things that were happening to us – like when Baby P seemed so much hungrier when I traveled out of town. And I REALLY wish I had known about this early on, when the doctor was worried that Baby P wasn’t gaining weight.

That’s why I’ll never use Medela bottles to measure milk for my baby again. I encourage you to test your bottles too and let me know what you find!

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A few notes, for those who really like to get into the weeds:

– I tested 3 different Medela bottles and 3 different Avent bottles and got the same results.

– I also tested the measurements using my Pyrex kitchen measuring cups and found the same thing.

pyrex measuring cup test

– I have also been using the Medela freezer storage bags all along and never noticed a discrepancy, so I have to assume their measurements are wrong too. Maybe one of you will test it and let us all know 🙂

– I have not received any kind of payment or free stuff or anything from Avent (or Medela, for that matter). I have not included any affiliate links in this post. I really just want to get the word out to other moms about this!


  1. Avery''s Mommom says:

    Your post had me a little worried so I immediately checked my Medela bottles. I have two variations (1/2 from 2010, 1/2 from 2015) of the 5oz (marked) bottles and two 8oz bottles (both from 2015). I checked them all and they all measure spot on. I also used a Pyrex measuring cup. Very strange. I don’t use the Medela storage bags anymore because I donate my milk, but I do recall seeing “approx” written on the bags in the past.

    I’m curious, have you have contacted Medela and informed them of the discrepancy? I have contacted them twice due to lids cracking and they immediately sent repacements.

    • brainybusinessbabe says:

      That was fast! Thanks for checking.

      Did you compare your Medela bottles to something that you were confident was accurate? It would have been hard for me to tell the difference in the Pyrex cups if I didn’t have the Avent one (which measured correct to the mL) next to it.

      Good idea about contacting Medela, I’m going to call and ask for a refund!

  2. Cate Rosales says:

    First of all, you are funny! The blue breastmilk made me laugh haha. I had no idea that they’re measurements were off, and that’s pretty awesome that you brought this to the attention of tired mamas everywhere!

  3. Kendra says:

    Yes!!!! I’m not crazy! I have a ton of Medela bottles from when my 1st was in the NICU; I eb and use Dr. Brown for my little man, but pump into whatever I have clean. I noticed the same difference when pouring from Medela into Dr. Brown. 🙁 This really bothers me as I’ve been tracking what I pump and what he eats since he was born a month ago.

  4. CHELSEA says:

    I’ve noticed the difference in measurements too by at least half an ounce. I’ve tracked how much my lo eats and how much I pump since birth and I’ve been getting pretty frustrated that the measurements are different between all my bottle brands (parents choice, aventis, dr brown, nuke, medella, lanolin, and bornfree). I only measure using one brand but I dont really know how accurate it is. I’ll have to compare using the pyrex measuring cup to see which bottles measurements are accurate and which ones aren’t.

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