Moving your body is not what dries up your milk
Where the workout-and-supply myth came from, what the research actually found, and the five things that genuinely do move the needle.
Quick answer
No. Moderate exercise does not reduce your milk supply. The American College of Obstetricians and Gynecologists states plainly that exercise does not adversely affect the quantity or composition of breast milk, and the NHS encourages new mums to build activity back up once they feel ready. Supply is driven mainly by how often and how well milk is removed from the breast. The things that genuinely can dent it during a training week are under-eating, under-drinking and skipping or delaying feeds, not the movement itself.
Of moderate-intensity activity per week is the level ACOG encourages postpartum women to work back towards.
The usual timing of the postnatal check, when most mums are cleared to progress activity (NHS).
Of mums still had diastasis recti at 12 months postpartum, which is why many keep training well into the feeding year (Sperstad et al., British Journal of Sports Medicine, 2016).
Where the myth came from
This one is unusual because it has a real research origin rather than being pure folklore. Early studies looked at lactic acid in breast milk after exercise and found it could rise temporarily following maximal-intensity effort, the kind where you are working flat out to exhaustion. A small number of infants in those studies were reported to feed less enthusiastically straight afterwards.
Two things then happened. The finding was repeated far more confidently than the evidence justified, and the crucial detail, maximal intensity, fell off along the way. What survived in mums' groups and comment sections was a much broader claim: that working out spoils your milk.
Later work has been reassuring. The consensus reflected in current obstetric guidance is that moderate exercise does not meaningfully change milk volume, milk composition or infant growth. It is worth saying clearly that a gentle core session during the baby's nap time is nowhere near the intensity those early studies were testing.
What actually controls your supply
Milk production runs largely on supply and demand. Frequent, effective removal of milk signals your body to make more, and infrequent removal signals it to make less. That mechanism does not have a clause about squats.
Where exercise can have an indirect effect is through the habits that come with it. If a new training routine means you push a feed back by an hour, eat noticeably less, or forget to drink across a busy day, supply can wobble. The cause there is the missed feed or the missed meal, not the exercise. Our guide to eating for steady energy while breastfeeding covers the fuelling side in more detail.
| Factor | Real effect on supply | What to do |
|---|---|---|
| Moderate exercise itself | No meaningful effect on volume or composition (ACOG) | Train as you feel able once cleared |
| Skipping or delaying feeds | Genuinely reduces the demand signal | Feed or express before you train |
| Eating too little | Can affect supply if intake drops sharply | Do not combine hard restriction with new training |
| Dehydration | Can reduce output if significant | Drink to thirst, keep a bottle where you train |
| Broken sleep and stress | Affects let-down and how you feel more than production | Shorter sessions, lower ambition, more consistency |
How to protect your milk supply while you exercise
None of this is complicated. Five habits cover almost all of it.
Step 1: Feed or express first
Training with a full chest is uncomfortable and makes it likelier that you will cut the session short or push the next feed back. Emptying first solves both problems at once.
Step 2: Wear a bra that actually fits now
Your pre-pregnancy size is rarely the right one while feeding. A supportive, correctly fitted bra makes movement comfortable, and comfort is what makes you do it again tomorrow.
Step 3: Eat before, not just after
Milk production carries a real energy cost on top of everything else. Pairing new training with a sharp cut in food is the single most common way mums end up feeling wrecked and blaming the exercise.
Step 4: Keep water within reach
Drink to thirst rather than to a rule. Most feeding mums are thirstier than usual anyway, and having a bottle beside your mat removes the friction.
Step 5: Build intensity gradually
Start with breathing and deep-core work, add gentle strength, and leave running and jumping until later. This is standard postpartum progression, not a breastfeeding rule, and it is laid out in our guide to deep core breathing, the first postpartum exercise to master. If impact is your goal, when it is safe to run again after having a baby is the next step.
The honest summary
Exercise is not the threat to your supply. Under-fuelling, under-drinking and missed feeds are. If you feed first, eat properly and build up gradually, there is no evidence-based reason to postpone gentle postpartum training until you have weaned.
Speak to your GP, midwife, health visitor or a lactation consultant if your supply drops noticeably, if your baby seems unsettled at the breast, or if you have pain, a blocked duct or signs of mastitis. Results and experiences vary from mum to mum.
So can you start rebuilding your core now?
For most mums who have had their postnatal check, yes. We have weighed the timing question up properly, including the argument for waiting, in is it worth starting a postpartum core program while you are still breastfeeding. The short version is that the feeding year is long, and an abdominal gap does not reliably close on its own while you wait it out.
If exhaustion rather than milk is the real barrier, beating new-mum fatigue without more coffee and the 15-minute nap-time approach are more useful reads than any training plan.
Short sessions that fit around feeds
Core Reconnect is a one-time digital guide, no subscription and no equipment, built around 10 to 15 minute deep-core sessions you can do on the floor between feeds. Use code GLOW20 for 20% off.
Frequently asked questions
Does exercise make breast milk taste sour?
Early research found lactic acid could rise temporarily after maximal-intensity exercise, and some infants fed less eagerly straight after. That intensity is far above typical postpartum core or strength work, and feeding before you train avoids the question entirely.
Do I need to eat more if I am exercising while breastfeeding?
Most mums need more energy while feeding than they did before pregnancy, and adding training increases that further. The practical advice is not to combine a new training routine with a sharp reduction in food. Speak to your GP or a dietitian for individual advice.
Can heavy lifting reduce my supply?
There is no good evidence that strength training reduces milk production. What matters more is building load gradually so your pelvic floor and abdominal wall keep up, particularly in the first few months.
Should I pump before or after a workout?
Before is usually more comfortable and keeps the feed rhythm intact. If your baby's timing means after works better, that is fine too, as long as feeds are not routinely being delayed or skipped.
How long after birth can I start exercising while breastfeeding?
Gentle breathing work is often encouraged early, and most mums progress activity after the postnatal check at around six weeks. After a caesarean the timeline is usually longer. Follow your own care provider's advice rather than a general timeline.
Sources: American College of Obstetricians and Gynecologists (physical activity and exercise during pregnancy and the postpartum period); NHS (breastfeeding and keeping fit and healthy with a baby); Sperstad JB et al., British Journal of Sports Medicine, 2016.
This article is general education and not medical advice. Results and experiences vary from mum to mum. Please speak to your GP, midwife, health visitor, lactation consultant or a women's health physiotherapist about your own feeding and exercise, particularly if you notice a change in supply or have any symptoms that concern you.