I looked it up and found out the following - I didn't realise everybody can be using different methods - how confusing!!!
The three basic ways to calculate trimesters:
There are basically three ways of dividing up a pregnancy into the three trimesters; they are by: Development, Gestation, Conception. They give different dates for when the second trimester begins and for when the third trimester begins. Your health care provider might prefer to use the Development method, while another Midwife or Doctor may use the Gestation method. I am sure they do that just to confuse pregnant women.
Development:
This uses actual developmental stages to divide up a pregnancy. From LMP to 12 weeks the embryo develops all the major organs and becomes a fetus. From 12 weeks to 27 weeks the fetus continues developing and reaches viability. From 27 weeks on the fetus finishes development and prepares for delivery. Here the second trimester begins at 12w 0d and the third at 27w 0d.
Gestation:
With this method you take the 40 weeks of gestation and divide by three. Here the second trimester begins at 13w 3d and the third at 26w 6d.
Conception:
This method is where you take the 38 weeks of post conception development, divide by three, and add two weeks. Here the second trimester begins at 14w 5d and the third at 27w 3d.
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