Second generation of mice showed reduced fertility after mothers spent 42 days on the ISS

The pregnancies began after the mice landed, so nothing in this experiment was gestated in orbit ^[26]. The exposure came before conception, so whatever the flight did passed through the mother, and the deficits appeared in animals that had never left the ground ^[26].

A mouse cycles every four to five days, against an average 28 days for a human menstrual cycle, Lane Christenson told Live Science ^[14]. Forty-two days on the station is about nine mouse cycles, and nine cycles at 28 days each is roughly 260 days of human cycling ^[25]. That arithmetic only lines up cycle counts. "But a lot of the molecular background is very similar" in how hormones fluctuate each cycle, Christenson said ^[15].

The one percentage figure reported for the second generation concerns muscle. Offspring of the flown mice had about 15% less muscle strength than the control groups' offspring ^[5], and the females among them were more stress-prone in a behavioral test ^[6]. "The offspring of those that came back from space were altered in their behavior, their muscle, their ability to reproduce further," said Christenson, a co-author and a professor of cell biology and physiology at the University of Kansas Medical Center ^[10][13]. He added that "just about every marker we looked at had some differences" ^[11]. Of the exposure length he said: "We were pretty surprised that a relatively short period of time, 42 days, was able to induce these striking changes" ^[12].

On the reproductive endpoints the published account is comparative and not numeric ^[7]. A hormone assay is an earlier and cheaper readout than a breeding trial, and anti-Mullerian hormone correlates with egg reserve without counting a single pup ^[8].

Christenson's theory for the pattern is an "acceleration of aging" set off by the predecessors' exposure to microgravity ^[16]. Earlier mouse and human-tissue work has suggested that the absence of gravitational force leaves cells more vulnerable to stress, with mitochondrial dysfunction implicated ^[17]. The comparison here is flown animals against Earth-bound groups ^[28]. That design supports attributing the effect to the flight as a whole, not to any one condition inside it.

Live Science reported that experts it consulted regard mice as good models for spaceflight health, being mammals with a reproductive system similar to ours, while cautioning that the carry-over to human astronauts is hard to judge from mouse data ^[22]. The human record it cites is thin. The NASA Twins Study compared astronaut Scott Kelly, after a year on the ISS, with his identical twin Mark, who stayed on Earth, and that line of work has suggested short-term flight has modest health effects ^[20]. Christenson said the new study was done to "fill a void" in research on how space travel affects the female reproductive system ^[19], and the authors argue for more work on long-duration flight and reproduction ^[23]. Few studies have examined the long-term health effects of spaceflight, including those touching reproduction ^[21].