Around 3-5% of babies are born on the guess date and I believe something around 40% are born within the two weeks before that, and 40% in the two weeks after that... but not sure of finer details like if they were left to birth naturally, induced etc. Gestation for first time babies is often longer than subsequent too. So if you really like her as a doula, I wouldn't worry too much. If it was closer maybe so, but like the others have said, not many, especially first time mothers, go two weeks or more earlier. Pre-labour and labour in general is often longer too.