A full term pregnancy lasts 40 weeks (280 days), and is divided into three time periods of about 13 1/3 weeks or 3 calendar months known as trimesters. Each trimester serves as an important milestone in the development of the fetus and progress of the pregnancy.

During the first trimester the sex of the baby is determined, the baby's organs are created, and twins may be formed.

You may experience fatigue, nausea, headaches, and swollen or tender breasts.  Your blood pressure will decrease and your heart rate will begin to increase as your body begins to accommodate the needs of your developing baby [2].

You may expect to gain 4 to 5 pounds during the first trimester unless you have significant nausea and vomiting. An additional 300 Calories per day are recommended during a normal singleton pregnancy.

Gestational Week 1 and 2 :

During the first two weeks after your last menstrual period egg follicles in the ovaries mature under the stimulus of follicle-stimulating hormone (FSH) a hormone secreted by the pituitary gland in the brain. High levels of another hormone estradiol, produced by the developing egg follicle,  cause secretion of luteinizing hormone (LH) ,yet another hormone from the pituitary gland. LH causes release of the egg from its follicle (ovulation) around day 14 of the menstrual cycle. Some women may experience slight pain with ovulation (mittle schmertz).

The empty egg follicle develops into a yellow colored structure called the corpus luteum (CL). The corpus luteum secretes progesterone which prepares the lining of the uterus for implantation by the embryo and causes a rise in basal body temperature. If the released egg is fertilized, then the fertilized egg will begin producing the hormone human chorionic gonadotropin which will maintain the corpus luteum until the placenta is able to produce enough progesterone to support the pregnancy.

Your due date may be estimated by adding seven days to the first day of your last menstrual period (LMP) and then subtracting three months. This method of estimating your due date depends on accurate recall of your last menstrual period, assumes you have regular 28 day cycles, and that ovulation and fertilization occurred on day 14 of your cycle. If you do not have regular menstrual cycles, or you do not ovulate when expected a more accurate estimation of your due date may be established by using ultrasound to measure the size of your baby before 20 weeks. [1]

The duration of pregnancy, called the gestational age or menstrual age, is expressed as completed weeks since the first day of your last menstrual period. For example a calculated age of 25 weeks and 2 days or 25 and 6 days from the first day of the last menstrual period would both be considered to be 25 completed weeks [3]. Conceptual age is a term used by persons who study the development of the embryo and fetus. The conceptual age is the time elapsed since ovulation and fertilization of the egg. Since the first day of ovulation occurs about 2 weeks after the first day of the LMP, the conceptual age is 2 weeks less than the gestational age.

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