When Does First Trimester End?
The first trimester starts with the first day of your last menstrual period, and ends roughly 13 or 14 weeks later. Three different methods are used to divide up a pregnancy into the three trimesters. Each methods results in slightly different spans for the trimesters.
The development method uses the developmental stages to divide up a pregnancy. The second trimester begins at 12 weeks and the third at 27 weeks.
From LMP to 12 weeks, the embryo develops all the major organs.
From 12 weeks to 27 weeks the fetus continues developing . At 27 weeks, your baby is considered viable.
From 27 weeks on, your baby gets matures and prepares for delivery.
The gestation method divides the 40 weeks of gestation into three equal stages. Using the gestation method, the second trimester begins at 13 weeks 3 days and the third at 26 weeks 6 days.
The conception method splits the 38 weeks after conception development into the three equal trimesters. The two week before conception become part of the first trimester. With this method, the second trimester begins at 14 weeks 5 days and the third at 27 weeks 3 days.