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Although most people think of pregnancy as lasting nine months, a full term pregnancy is closer to ten months -- 38 to 40 weeks. The official start of pregnancy is the first day of a woman’s last menstrual period (LMP), which is usually three to four weeks before she is “officially” pregnant. Each of the three trimesters or stages of pregnancy is about 13 weeks long. Each stage consists of specific medical and physiological developments.

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1. Before Conception

Each month, the ovary releases a group of eggs, oocytes, for ovulation. These eggs develop within small fluid-filled sacs called follicles. In most cases, only one egg matures and becomes a dominant follicle. It suppresses the other follicles, which deteriorate and stop growing. Approximately two weeks before the woman’s menstrual cycle begins, the surviving follicle opens, releasing the egg from the ovary during the ovulation process. Once ovulation occurs, the ruptured follicle becomes the corpus luteum, a structure that secretes estrogen. It also secretes progesterone, a steroid that helps prepare the lining of the uterus for the implanting of the embryo.

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This site offers information designed for educational purposes only. You should not rely on any information on this site as a substitute for professional medical advice, diagnosis, treatment, or as a substitute for, professional counseling care, advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a physician or other health-care professional.