Guesstimating is not the right answer
My wife had been estimating the length and interval between contractions and thought they were 30-35 seconds and 7-8 minutes apart. When I got home I grabbed this app and we started timing. They were 1:15 - 1:35 in length and 4 minutes apart. We got to the birth center 3 minutes before her water broke AND she was 10 centimeters. This app saved our car from needing a deep detailing and got us there just in time for everything to go amazingly.
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Contraction Timer Lite