Abortion Methods: Trimester 1

--Medical Methods

--Surgical Methods

Fetal Development: Trimester 2

Fetal Development: Trimester 3

Female Reproductive Cycle

Stages of Pregnancy and Fetal Development Introduction

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The first trimester of pregnancy is the trimester with the most developmental changes of the fetus.

Month 1 (see figure)

Implantation: Five to seven days after the sperm fertilizes the egg, the egg implants in the uterus and the embryo begins to grow and develop.(2) The corpus luteum of pregnancy secretes progesterone, maintaining the thickened endometrium (see Female Reproductive Cycle page).(3) The primary yolk sac begins to form,(3) along with the amniotic sac and tissues that give rise to the placenta and umbilical cord.(2)

Gastrulation: The embryo becomes multilayered through the process of gastrulation,(3) and by the 5th week of the pregnancy the embryo has an outer layer (originally the ectoderm) consisting of the brain, nerves, and skin. The middle layer (originally the mesoderm) becomes the bones, muscles, blood vessels, heart, and sex organs. The inner layer (originally the endoderm) holds the stomach, liver, intestines, lungs, and urinary tract.(2)

Body Plan: Through the process of neurulation, the embryo develops a spinal cord. The vertebral column begins to be laid down, and by the end of the 6th week of pregnancy the embryo has a head and trunk of the body.(2)

Organ Systems: The eyes, limb buds, and other features begin to form, along with the heart, which begins pumping at 25 days after conception.(2)

By the 6th week of pregnancy, the embryo is approximately half-an-inch long and weighs a fraction of an ounce.(2)

Month 2 (see figure)
The second month of pregnancy involves major developmental solidification of the organ systems and body of the embryo, such that by the end the embryo looks like a tiny human newborn.

Body Plan: The trunk of the embryo elongates and straightens,(3) and by the end of the second month the embryo is a little longer than one inch and still weighs less than an ounce.(2)

Organ Systems: All of the major organ and body systems have begun to develop, including the brain, lungs, liver, and stomach.(2) The first bone cells are appearing at this time, along with limbs and features such as the fingers, toes, and joints.(2) The inner ear is beginning to form, as are components of the eye.(3)

The embryo is now called a fetus.

Month 3 (see figure)
By the end of the third month of pregnancy, the fetus is completely formed.

Body Plan and External Features: The head is large, fingers and toes are distinct, external genitalia begin to differentiate by sex.(3) The fetus may have begun moving its arms, legs, and head.(2)

Organ Systems: Beating at 120 to 160 beats per minutes, the heart of the fetus has four chambers. Kidneys are becoming functional, and intestines have begun to develop. The umbilical cord is now fully formed.(2)

By the end of the first trimester, the fetus is about four inches long and weighs just over an ounce.(2)