Becuase shit happens. Miscarriages and the like. Women don't tell any one till well into the pregnancy.
Here's the other dark shit that really happened.
The Doctor did not tell Captain Janeway, and that's the betrayal that would have really gotten Kathryn's goat.
Imagine if your answer phone started fucking with you, deciding which messages you should and should not receive on ethical grounds or because they think you're too much of a wussy to handle the truth...
A 1/4 of her crew are terrorists planning to slit her throat while sleeping but there's a lightbulb keeping medical privilege about their psyche evaluations in conflict with ship security... And probably Tom Paris' STD panel.
The Doctor's program would have been gutted.