 Why didn't the Lieutenant know she was pregnant?
Once every two hours, the artificial satellite looped completely around the planet, watching what went on below. According to the major, the baby was due sometime around the first of September. And then, only half an hour ago, a meteor had hit the radar room. The depressurization hadn't hurt her too much, but the shock had been enough to start labor. But as soon as we get within radio range of White Sands, I'll ask them to send up an emergency rocket with an incubator. Major Banes was in the Communications Center a full five minutes before the coastline of California appeared on the curved horizon of the globe beneath them.
 She has an irregular cycle.
 She has an irregular cycle.  She had her tubes tied before going into space.  She wasn't keeping track of her cycle.  Women don't have periods in space.