Specific DNA changes make the mammary glands in pregnant mice gear up for milk production faster if the animals have been pregnant before.

Credit: Camila dos Santos, Cold Spring Harbor Lab

Many women report that breast-feeding becomes easier with each pregnancy. To find out why, Gregory Hannon at Cold Spring Harbor Laboratory in New York and his colleagues gave pregnancy hormones to female mice and monitored changes in their mammary glands over time.

In mice that had been pregnant once before, the milk-producing ducts expanded in number more quickly (pictured, right) and generated milk proteins sooner than the ducts of mice that were first-time mothers (pictured, left). Genetic analysis of mammary-gland cells after the animals had stopped lactating showed that a previous pregnancy resulted in a long-term loss of methyl groups from DNA in genes that are activated during lactation.

This epigenetic 'memory' primes these genes for fast reactivation in a subsequent pregnancy, the authors say.

Cell Rep. http://doi.org/4j2 (2015)