What are the weeks for the first, second, and third trimesters of pregnancy?

Generally, the first trimester refers to weeks 1–12 of pregnancy, the second trimester to weeks 13–27, and the third trimester from week 28 until delivery. If there are any concerns, it is recommended to seek medical advice early. The detailed breakdown is as follows:

The first trimester, spanning from conception to the end of week 12, is a critical period for fetal organ development. During this time, the fertilized egg gradually develops into an embryo and then into a fetus, with basic organs such as the heart, brain, and limbs forming progressively. As embryo implantation is still unstable, pregnant women should pay special attention to rest, avoid strenuous activities, and undergo their first prenatal checkup to establish a maternal health record and screen for early developmental abnormalities.

The second trimester covers weeks 13 to 27. Fetal growth accelerates during this period, and organ functions gradually mature. Pregnant women typically experience noticeable abdominal enlargement, and early pregnancy symptoms often subside. This stage is relatively stable, allowing for a moderate increase in physical activity. Regular ultrasound examinations (such as anomaly scans) are necessary to monitor fetal growth and development. Additionally, close attention should be paid to indicators like blood glucose and blood pressure to prevent pregnancy-related complications. The third trimester begins at week 28 and continues until childbirth. During this phase, the fetus gains weight rapidly, and the fetal position gradually becomes fixed.

It is important to remember the gestational week divisions for each stage and perform appropriate care accordingly. If any abnormal symptoms occur—such as bleeding or abdominal pain—medical attention should be sought immediately, regardless of the current stage of pregnancy.