With high precision, Natural Cycles can calculate how many days you are into your pregnancy and predict your due date, as the app knows exactly which day you ovulated which is when conception took place. We can, therefore, calculate your due date with higher precision than for instance through ultrasound or by counting from your last period, since the baby could be larger or smaller than average and the fact that ovulation doesn't always occur exactly 14 days after the last period's first day.
If you suspect that Natural Cycles has found an incorrect ovulation day, simply to go Profile in your app and manually change your due date. This may occur if you have not measured enough around ovulation, or if your temperatures fluctuate very much.
What Natural Cycles does is that we count from your actual ovulation date and then add 14 days to your pregnancy to match the way your midwife counts.
The health care system assumes you've ovulated on CD 14, and therefore counts from the first day of your last period, which can be very incorrect - on average Natural Cycles users ovulate on CD 17.