After my second daughter was born, I was prepared to wait three or four days for my breast milk to come in, as I had with my first. But just as everyone had warned, things were completely different the second time around. Just half a day later, my body was already churning out the good stuff.

“She’s already got a milk mustache!” one of the nurses told me as she weighed my newborn. That early production was a welcome surprise, since I had a long, anxious wait for milk with my first baby. This time, my experience was definitely smoother. And while that’s definitely not true for every woman, I’ve since talked with many women who have also had an easier time nursing their second baby compared with their first.

Part of that ease probably comes from expertise. After breast-feeding a tiny, sleepy, floppy infant once, mothers have mastered the football hold, the cross over hold and the cradle hold. We know how to coax the baby to open her mouth. We can pick out the whispery sounds of swallowing. And we don’t hesitate to rip off a bad latch and make the baby try again. Breast-feeding is a skill we’ve learned, so it makes sense that the second time around can seem easier.

Now, a new study in mice hints at another reason why breast-feeding the second baby could be easier: Our bodies seem to remember how to make milk. After a first pregnancy, milk-producing mammary glands remain in a state of preparedness, ready to quickly spring into action and feed subsequent babies, Camila dos Santos of Cold Spring Harbor Laboratory in New York and colleagues write May 19 in Cell Reports.

In response to hormones circulating during pregnancy, DNA in mammary gland cells loses certain marks called methyl groups, the team found. Before pregnancy, these methyl groups seem to act like brakes that prevent certain milk-making proteins from being made. But once those brakes are gone, the milk machinery is free to kick into gear as mammary tissue undergoes a massive expansion (not surprising to any new mother), new ducts are formed and milk gets made.

Particular methyl groups removed by proteins during the first pregnancy stay off for subsequent pregnancies too, the researchers found. In this way, the mammary gland “remembers” the first pregnancy and starts preparing to make milk more quickly the second time around.

The researchers also found that in response to pregnancy hormones, the mammary glands of mice that had been pregnant before also grew more treelike branches of milk-moving ducts than mice who had not been pregnant before.  Those results may help explain why some women make more milk for their second kid, an observation that’s backed up by a 2001 Lancet study of 22 mothers. One week after birth, these mothers produced about 30 percent more milk the second time around than the first. And second babies spent less time nursing, too.

That second baby’s speed was definitely true for us. In her early days, my second babe spent no more than five minutes nursing at a time. She was so fast that I was worried she wasn’t getting enough, but her chubby little body helped convinced me otherwise.

It’s a fantastically efficient system: By the time a second baby arrives, a mother no longer has the luxury (ok, probably not the right word) of never-ending nursing sessions. She’s got other little ones to chase. And so her body helps out the best way it can, by making plentiful and speedy meals for the new arrival. 

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