Now that I’m nursing a newborn again, I’ve renewed my fascination with the science of breast milk. In particular, “lactocrine programming;” the idea that hormonal signals in Mama’s milk can “program” Baby’s body and behavior. So many new developments in the last few years since UJ was born!
Today’s question: Does breastfeeding make kids more fertile later in life?
Theories abound with evidence accumulating from various studies on humans and other mammals. Researchers Katie Hinde and Danielle LeMay, at Harvard and UC David respectively, offer some intriguing insights in their SPLASH! Milk Genomics blog.
The upshot: The first few days after birth are a crucial window for the development of reproductive tissues that will become part of the cervix, uterus, endometrium, and other parts. These tissues start growing when they receive signals from a hormone called relaxin (a multipurpose hormone, it also relaxes blood vessels and ligaments during pregnancy, opens the cervix, and much more).
There’s relaxin in the milk! report Hinde and LeMay, referring to a study on pig milk:
Here’s the crazy part: relaxin is delivered by the mother via her milk. Piglets that are allowed to suckle have relaxin in their blood stream, but not piglets fed a milk-replacer . Relaxin activity in pig milk is highest in the first few days of lactation, and is similar to findings from dogs and humans. Experimental manipulations have shown that as little as one colostrum feeding in the first 12 hours after birth can make a difference. For example, just a single colostrum feeding bout in the first hours after birth allows for typical cervical cell proliferation and development—an important predictor of future litter size.
But does access to milk really predict future fertility? It did in a recent pig study involving over 1,500 litters, say Hinde and LeMay:
Female pigs with limited access to maternal-origin hormones via milk as piglets had reduced litter size as adults. So, it is safe to conclude, at least in pigs, that the number of babies born in any generation was partly programmed by their grandmothers via milk hormones.
It’s mind-boggling, the possibility that access to breast milk in infancy — or at least colostrum (the thick milky fluid produced in the first days after birth, which also contains relaxin) may have an impact on our kids’ reproductive development.
Not that these preliminary (porcine) studies say anything conclusive about humans. For that, we’d need to address many more questions: Do women whose mothers never attempted breastfeeding have more fertility problems — or fewer kids — than their breastfed peers? Do they have a different growth trajectory in puberty? How does relaxin affect reproductive development in sons? Can breastfeed girls bear children later in life than those who weren’t breastfed? How long does a kid need to nurse to receive reproductive benefits — the first day after birth, a few days, the first week? More?
Further research is warranted. Even if turns out that fertility is only slightly enhanced among breastfed kids (that’s my bet, anyway), it’s more fuel for the breastfeeding movement.