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.
No matter what method is used; a pregnancy has a beginning, middle and end. You can use the trimester dates from your Doctor, a book or a website, whichever you feel most comfortable with.
What is the length of the first, second and third trimesters?
The length of the trimesters is often a source of confusion. The word trimester means 'three months'. Three periods of three months each, gives pregnancy a beginning, a middle and an end period.
These periods match the developmental stages to divide up a pregnancy. From LMP to 12 weeks the embryo develops all the major organs, becomes a fetus, and the placenta takes over control. 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.
When does the first trimester end or third trimester begin?Does it really matter exactly when you are out of the first trimester or start the third trimester? No. When the trimesters start and end depends on who you ask, which book you read or what website you visit. Most health care providers actually talk about your pregnancy in weeks, rather than months or trimesters.
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