Can CMV Pass Through Breast Milk? What Parents Should Know

The Bottom Line: For a healthy, full-term baby, breastfeeding is generally safe and encouraged even if you have had CMV. For a very premature, very low-birth-weight, or medically fragile baby, you will need to work with your Neonatal Intensive Care Unit (NICU) to decide if your breast milk should be used fresh, frozen, pasteurized, or temporarily replaced.

Yes, cytomegalovirus (CMV) can be passed to a baby through breast milk ^[1]. Up to 80% of mothers with a history of CMV will shed the virus in their breast milk at some point during lactation ^[2]. However, the level of concern depends mainly on your baby’s gestational age, birth weight, and overall health ^[3].

What Does Being “CMV-Positive” Mean?

When doctors say a mother is “CMV-positive,” they usually mean she is CMV-seropositive—meaning a blood test shows antibodies (IgG) from a past CMV infection. It does not necessarily mean you have an active illness ^[4].

Even if you have these antibodies, it does not mean every drop of your milk is infectious. Viral shedding in breast milk often peaks in the early weeks after birth and then declines ^[1].

Note on Timing: It is also important to distinguish between postnatal CMV (acquired after birth through milk) and congenital CMV (passed during pregnancy). If there is concern your baby might have congenital CMV, doctors usually need to test your baby’s saliva or urine within the first 21 days of life to reliably tell the difference ^[5].

Healthy, Full-Term Babies

If you have a history of CMV and your baby was born full-term and healthy, exposure to CMV through breast milk is generally not a reason to withhold your milk ^[6].

When healthy full-term infants acquire postnatal CMV, they rarely show any symptoms ^[7]. Their mature immune systems, combined with the protective antibodies passed to them through your breast milk, are typically strong enough to handle the virus. Because of this, medical guidelines generally encourage routine breastfeeding for full-term babies; the immense health and immune benefits far outweigh the very low risk of CMV complications ^[6][4]. (Note: If your full-term baby has a diagnosed immune disorder, always follow your doctor’s individualized advice.)

Premature and Very Low-Birth-Weight (VLBW) Babies

The risks are different for babies born very prematurely (especially before 32 weeks) or with a very low birth weight (under 1500 grams, or about 3.3 pounds) ^[3].

Clinically significant postnatal CMV disease is uncommon overall, but it is concentrated in these most immature infants ^[3]. Preterm infants have underdeveloped immune systems and missed out on the final weeks of pregnancy when mothers transfer the most protective antibodies. If a vulnerable preemie acquires CMV, they have a higher risk of developing severe symptoms ^[8]. These can include:

• Sepsis-like illness: A severe, body-wide inflammatory response ^[8].
• Respiratory issues: Such as pneumonia or worsening lung function ^[9].
• Hepatitis: Inflammation of the liver ^[8].
• Thrombocytopenia: Low levels of blood platelets (which help blood clot) ^[8].
• Some observational studies note an association with longer hospitalizations ^[10].

Because of these risks, your NICU care team will carefully weigh the benefits of your breast milk against the risk of CMV transmission, looking closely at your baby’s specific situation ^[6].

Managing the Risk in the NICU

If your baby is high-risk, the NICU may suggest treating your breast milk or using alternatives. Do not attempt to pasteurize, heat, or alter your breast milk at home. These are precise medical procedures that require specialized hospital equipment.

• Freezing and Thawing: Freezing milk (often at -20°C for a specific duration in the hospital) reduces the amount of live virus, though it does not completely eliminate the risk of transmission ^[11][12]. Some NICUs use this to balance nutritional benefits and infection risk ^[2].
• Pasteurization (Heat Treatment): Exposing the milk to controlled heat (such as standard Holder pasteurization at 62.5°C for 30 minutes) is highly effective at destroying the CMV virus ^[13][14]. However, heat treatment can also break down some of the beneficial immune properties in breast milk ^[15].
• Pasteurized Donor Human Milk or Formula: If your own milk cannot be used safely, the medical team may recommend pasteurized donor breast milk or formula ^[16][17]. Donor milk is often preferred to formula for very premature infants, but formula is also a safe alternative when donor milk is unsuitable or unavailable.

What to do while you wait:
If the NICU is deciding on a feeding plan, do not stop pumping. Continue to express, label, and store your milk exactly according to the hospital’s instructions. Pumping is crucial to establishing and maintaining your milk supply so that if and when your baby is ready for your milk, you will have it available.

It is completely normal to feel upset or disappointed if you are asked to delay feeding your baby your own milk. Remember that using donor milk or formula temporarily is not a failure, and it does not mean you won’t be able to provide breast milk later.