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Pregnancy is the development of one or more offspring known as an embryo or fetus inside woman's uterus. The length of pregnancy is generally about 40 weeks from the start of the last normal menstrual period which includes 2 weeks before conception.
Human pregnancy is arbitrarily divided into three trimester periods of three months each, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage. During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The third trimester is marked by further growth of the fetus and the development of fetal fat stores.
During pregnancy, the woman undergoes many physiological changes, including cardiovascular, hematologic, metabolic, renal and respiratory changes to ensure the fetus is provided for blood sugar, breathing and cardiac output. Levels of progesterone and oestrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and subsequently the menstrual cycle.
Usually after 14 days of the last normal menstrual period ovulation occurs where the egg cell is released from one of the female's two ovaries. This egg cell unites with male sperm in one of the two fallopian tubes; this process is referred as fertilization where the egg cell fuses with the male gamete. The fertilized egg, known as a zygote, then moves toward the uterus, a journey that can take up to a week to complete. Cell division begins approximately 24 to 36 hours after the male and female cells unite. Cell division continues at a rapid rate and the cells then develop into what is known as a blastocyst. The blastocyst arrives at the uterus and attaches to the uterine wall, a process known as implantation.
In first 10 weeks of gestation the cell divides rapidly and begin to differentiate into the various body systems, the basic outlines of the organ, body, and nervous systems are established. By the end of the embryonic stage, the beginnings of features such as fingers, eyes, mouth, and ears become visible, during this time, there is also development of structures important to support the embryo, including the placenta and umbilical cord. Placenta connects the developing embryo to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply while the umbilical cord is the connecting cord from the embryo or fetus to the placenta.
After about 10 weeks of gestational age, the embryo becomes known as a fetus when the risk of miscarriage decreases sharply, at this stage the length of a fetus is typically about 30 mm (1.2 inches) and the heart can be seen beating via ultrasound; the fetus can be seen making various involuntary motions at this stage. During continued fetal development, the early body systems and structures that were established in the embryonic stage continue to develop. Sex organs begin to appear during the third month of gestation. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs only in the last weeks of pregnancy.
Pregnancy is typically broken into three periods, or trimesters, each of about three months or about 14 weeks, resulting in a total duration of 42 weeks, although the average duration of pregnancy is actually about 40 weeks.
» First Trimester
» Second Trimester
» Third Trimester
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