If I understand it correctly, the My Days app uses only a calendar method of determining your fertile times, and if you've heard any jokes about the rhythm method, you'll know that it's not as effective as many other forms of birth control. Additionally, the pull-out method is not the most reliable. There's a lot more research on fertility that can be used that would give you a better idea of when your body is preparing to ovulate and when you have actually ovulated. Check out the charting to avoid thread for more information on that.
If you did actually ovulate when the app thinks you did and you were actually 10 DPO when you did the blood test, it is very unlikely that you are pregnant. Usually, implantation occurs on or before 10 DPO, and there would be at least a little bit of HCG by then to detect. However, I don't have a lot of trust in the app that it accurately determined your fertile days, so you can't really know for sure until your period starts or you get a positive test.
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