Final answer:
The first trimester of pregnancy consists of weeks 1 to 12, the second trimester includes weeks 13 to 28, and the third trimester spans from weeks 29 to birth (around week 40).
Step-by-step explanation:
The average duration of pregnancy is 40 weeks, counted from the first day of the last menstrual period, and it is usually divided into three trimesters. Each trimester has its distinct characteristics and developmental milestones.
First Trimester
The first trimester encompasses weeks 1 to 12. During this period, the basic structures of the fetus form, including the limb buds, heart, eyes, and liver. It's also when morning sickness is most common.
Second Trimester
The second trimester spans from weeks 13 to 28. Nausea and vomiting may subside, the pregnant person may feel more energized, and weight gain occurs. Fetal movements are often felt for the first time around week 20.
Third Trimester
The third trimester includes weeks 29 through to birth, around week 40. Rapid growth and weight gain occur for both the fetus and the mother. The fetus's movements become stronger, and symptoms like back pain, swelling, and increased urination can arise.