I wouldn't say that nobody noticed...Hetty knew, Arthur knew, Adam knew, and someone in Hetty's family knew since she was probably plagued with morning sickness and was also thickening around the middle.  However, these symptoms would probably have only been noticed by those who were close to Hetty since it was her first child. Besides, a fortnight is only two weeks, but all the traveling she was doing added to the stress she was under (pregnant by the man she loves who has abandoned her and being forced to marry someone she doesn't for the sake of her honor) probably caused her to go into early labor.   In this time period, and in some cases, even today, young women who are physically fit tend to not show until the later stages of pregnancy on the first child.   Hetty was definitely in this category since she worked hard as most young women did during the time this book was written and set.  It is only after the first child when the abdomen muscles are more slack and weaker that these same young women show earlier thus giving away the secrets earlier to the public.