Ingrid Christensen, the Norwegian distance runner, was puzzled one year when she finished in the middle of the field in the Norwegian national cross country championships that she usually won by a mile. Turned out she was 5 months pregnant and didn't even know it because she trained so hard she wasn't menstruating regularly. Anecdotal, of course, and in no way proof that your odds remain high even if you're training that hard. They're probably way lower if you're not menstruating, dehydrated, anemic, or generally exhausted. But they're not 0% even then. Oh, and the kid was perfectly healthy. But I'd say Lise prob'ly knows this stuff pretty well (aren't you a midwife, Lise?) and she says your chances shouldn't be affected if you're training somewhere under that ceasing-to-menstruate threshold. Good luck, and have fun!