How breasts produce milk in pregnancy and beyond

How will my breasts change in pregnancy?

Your breasts begin to gear up for feeding your baby as soon as you're pregnant. Having tingling nipples and tender, swollen breasts is one of the first signs of pregnancy (Murray and Hassall 2014, NHS 2016a). It's caused by hormones surging through your body.

The skin around your nipples (areolas) may also appear darker, and have tiny bumps. This is nature's visual way of directing your newborn towards her feed.

The tiny bumps around your areolas produce an oily substance that cleanses, lubricates and protects your nipples from infection during breastfeeding (Geddes 2007, Welford 2011). The substance smells like amniotic fluid, so your baby will instinctively move towards this familiar smell soon after birth (Doucet et al 2012, Sullivan et al 2011).

By the time your baby is born, the milk-making tissues in your breasts, called mammary glands, may have doubled in size. This is a gradual change that varies from woman to woman. It can happen in mid or late-pregnancy, or even after you've given birth (Geddes 2007).

There's no link between how big your breasts become during pregnancy and your ability to produce milk once your baby is born (González 2014, Pickett 2016). When your milk comes in a few days after your baby's birth, your breasts will look and feel heavier and fuller.

How do my breasts produce milk?

Mammary glands in your breasts produce breastmilk. Within each mammary gland, different parts play a role:

• Alveoli: where breastmilk is produced. These clusters of small grape-like sacs are surrounded by tiny muscles that squeeze them to push milk through your breast to your baby. Alveoli develop during each pregnancy (Arendt and Kuperwasser 2015).
• Ductules: small, branching canals that carry milk from the alveoli to the main milk ducts.
• Milk ducts: these carry milk from the ductules straight to your baby via your nipple. More of these ducts grow during each pregnancy (Arendt and Kuperwasser 2015). The average is nine or so in each breast by the time you start breastfeeding.
  (Geddes 2007, Inch 2014)

You may leak a few drops of milk during your pregnancy. That's because your breasts are getting ready to make milk during your second trimester (Geddes 2007, Inch 2014, Welford 2011). You'll be able to breastfeed your baby even if she arrives prematurely.

After your baby is born, and you have delivered the placenta, your levels of oestrogen and progesterone start to drop. This makes way for the hormone prolactin to be released from the pituitary gland in your brain (Inch 2014).

Prolactin tells your body to make lots of milk to nourish your baby, and may also help you to feel more loving and protective towards your baby.

When can I start feeding my baby?

You can start feeding your baby as soon as you're both ready after the birth. Having a skin-to-skin cuddle helps:

• Your body to release the "love hormone" oxytocin, which is great for breastfeeding and bonding.
• To relax your baby and calm her heart rate.
• To keep your baby warm and help her feel safe now she's no longer in your womb.
  (Buckley 2015, Hubbard and Gattman 2017, Pickett 2016)

All these elements work together to get breastfeeding off to a great start.

The first milk you'll feed your baby is colostrum, which is a concentrated, creamy-looking, high-protein, low-fat substance (Gidrewicz and Fenton 2014). This is exactly what your baby needs in her first three days of her life.

Colostrum is full of disease-fighting cells and proteins that strengthen your baby's immune system (Ballard and Morrow 2013, Palmeira and Carneiro-Sampaio 2016).

Colostrum also contains unique ingredients that prompt your baby's growth and populate her gut with microbes. These microbes help your baby’s intestines to fight the germs that cause diarrhoea and other infections (Palmeira and Carneiro-Sampaio 2016).

About three days after your baby has had this boost from colostrum, your milk comes in. Your breastmilk has all the germ-fighting and microbial ingredients that colostrum has. As well as these, it has different protein and fat levels that are perfectly tailored to help your baby grow at the healthiest rate (Ballard and Morrow 2013).

Your breastmilk is the only food and drink your baby needs for about the next six months, but your baby will benefit most if you can breastfeed him for his first two years (Ballard and Morrow 2013, WHO nd).

What happens when my baby starts to feed?

For your baby to get your milk, it must be released from the alveoli, which is called letdown. Here's how it happens:

• As your baby suckles, the sensation in your nipple causes another part of your pituitary gland to release oxytocin into your bloodstream.
• When oxytocin reaches your breast, it causes the tiny muscles around the milk-filled alveoli to contract and squeeze, and release milk.
• Your milk moves along to the ducts just below the areola.
• When your baby feeds, she presses the milk from the ducts into her mouth. (Welford 2011)

As your milk flow increases during letdown, you may also feel some tingling, stinging, burning or prickling in your breasts (NCT 2012, Walker 2011, Welford 2011). This is normal, and the sensation passes in a few moments. The tingling reduces the longer you breastfeed, so you’ll notice it less and less over the coming weeks (Welford 2011).

You may find that your milk drips or even sprays as it lets down. This is because, at first, your breasts may make more than enough milk for your baby.

It can be normal to have very full breasts during the first few days. Feeding your baby often, and for as long as she wants, will help to soften your breasts. Feeding whenever your baby wants is called responsive feeding and it's what your breasts are designed for. If your baby keeps taking milk from your breasts, your body will soon learn how much to make for your baby (NHS 2016b). This will stop your breasts from being over-full.

You may feel some contractions in your belly during the first few days as your baby suckles. These after-pains may feel like mild labour contractions (NCT 2012, NHS 2018). It's oxytocin getting to work again, shrinking your womb back to its pre-pregnancy size.

If you need pain relief for afterpains, ibuprofen works better than paracetamol (Deussen et al 2011). Both are safe to take while you're breastfeeding (Jones 2017).

How do my breasts know how much milk to make?

While your baby is feeding, her suckling stimulates your brain to release more prolactin. Prolactin tells your breast to make more milk to replace the milk she's having. Your breasts then have a pre-order of milk ready for your baby's next feed.

The more often your baby breastfeeds, the more the prolactin levels in your bloodstream are boosted to produce milk. Prolactin is also the reason that your periods stop, as it can suppress ovulation.

Some mums use breastfeeding as a form of birth control for the first six months, though this method isn't fail-safe (FPA 2014).

Over the weeks, your body gets so familiar with how much milk your baby needs that it depends less on prolactin to trigger milk production. Your levels of prolactin decrease, and eventually your periods return.

You will still have plenty of milk, though. Lower levels of prolactin are enough to make all the milk your baby needs (Kent 2007, Welford 2011). By this time, your baby's feeding patterns become the main trigger for making milk. This is because of another hormone called feedback inhibitor of lactation (FIL).

FIL tells each breast how much milk to produce. If your baby feeds from a breast often, the levels of FIL in that breast will be low. This acts as a signal for that breast to make more milk (Kent 2007, Welford 2011).

If milk isn't removed very often, perhaps because your baby is still learning to suck well, or you are using formula to top up breastfeeds, milk will stay in your breasts for longer. As a result, FIL will build and milk production slows (Kent 2007, Welford 2011).

You could express milk to boost your supply. As long as milk is removed from your breasts, FIL stays low enough to allow milk-making to continue or increase.

FIL acts separately in each breast. Over time you may notice that one breast gets fuller and produces more milk than the other, or that you or your baby prefer feeding from one breast over the other (Kent 2007).

This is very common and seems to happen most often with the right breast. This is regardless of whether you are left or right-handed, or whether you consistently alternate the breast that you feed from first (Kent 2007).

FIL is also the reason why one of your breasts can produce enough milk to fully feed your baby. So if you have twins, it's possible to feed a baby on each breast.

Discover more fascinating facts about breastfeeding.