Over the course of the nine month pregnancy, the fetus develops through three trimesters. In the first trimester, the zygote implants in the uterus and rapidly divides, with major organs beginning to form by week 5. The fetus' limbs, fingers and toes develop, and it begins to move independently by the end of the trimester. In the second trimester, the fetus grows significantly as its organs continue to mature and develop. By week 20 the mother may feel movements, and sex can be determined. The third trimester sees further growth and neurological development, with the fetus considered full term by week 37 and able to survive outside the womb by week 34.

1. Pharmacist Grab Bag Question Carly Green

2. Can you explain how a fetus develops over the nine month pregnancy?

3. First Trimester The first two weeks of pregnancy is actually before conception happens. The due date for a woman is calculated by counting ahead 40 weeks from the start of her last period. Conception typically happens on two weeks after her last period starts. Week three is when the woman actually becomes pregnant. The sperm and egg join together to form a zygote. It then travels down the fallopian tube towards the woman’s uterus. The zygote will begin rapidly dividing with the inner cells becoming the embryo. Week four is when the zygote which is now known as a blastocyst reaches the woman’s uterus. It will burrow into the uterine wall for nourishment. During this week, the placenta begins to form. A woman may receive a positive pregnancy test now.

4. First Trimester Week five is when the baby’s brain, spinal cord, heart, and other organs form. The embryo is now made of three layers which are the ectoderm, mesoderm, and endoderm. The baby’s heart and a primitive circulatory system form in the mesoderm. The baby’s heart is beating at a steady rhythm. The baby’s skeleton is also beginning to form. The baby is between 1/16 to 1/18 inch long which is about the size of the tip of a pen.

5. First Trimester Week six is when the neural tube along the baby’s back closes and the heart is pumping blood. Basic facial features appear and the body becomes C-shaped. The hand and feet buds have webbed-like structures. A vaginal ultrasound could detect an audible heartbeat now. Week seven the baby’s brain and face are developing rapidly. Tiny nostrils become visible, and the eye lenses begin to form. The arm buds from week six are now in the shape of paddles. The baby currently weighs less than an aspirin. In week eight, the baby’s arms and legs have grown longer and fingers and toes have formed but are still webbed. The upper lip and nose have formed. The trunk of the body is straightening. The baby may begin to move, but it will not be felt by the mother. The bones are beginning to form, and the muscles can contract.

6. First Trimester In week nine, the baby’s arms grow, develop bones, and bend at the elbows. Toes begin to form, and the baby’s eyelids and ears continuing developing. All the baby’s joints are working and allowing the baby to move freely. The heart has divided into four chambers, and the valves are beginning to develop. The baby is now the size of a grape. Week ten the baby’s head becomes more round and the neck starts to develop. The majority of their vital organs, kidneys, liver, brain, and lungs are functioning, but will continue to develop. The fingers are no longer webbed. The baby is actively swallowing amniotic fluid and kicking his legs.

7. First Trimester Week eleven, both the fingers and the toes have separated by week 11. The baby’s skin is still very transparent, but his bones are beginning to harden. The external genitalia have almost completely formed. In week twelve, the baby will begin to take on more of a normal appearance as the eyes move closer together and the ears move closer to their final position. The intestines have grown so fast that they actually extend into the umbilical cord, but they will begin to move back into the abdomen. The kidneys can now secrete urine, and the nervous system is continuing to mature. The baby may have developed more complicated reflexes such as sucking.

8. First Trimester In week thirteen, fingerprints are now located on the tips of the baby’s fingers. The kidney and urinary tract are completely functional which allows the amniotic fluid he has been swallowing to be excreted. Covering the baby’s skin is a fine, soft hair called lanugo .

9. Second Trimester In week fourteen, the baby’s arms are going to begin to lengthen and be more proportioned to the rest of the body. The liver is beginning to produce bile, while the spleen is beginning to produce red blood cells. The development of your baby’s brain enables him to use his facial muscles to grimace, frown, and squint. He may even be sucking his thumb. In week fifteen, the baby’s skin is very thin, and his blood vessels can actually be seen through the skin. The baby’s bones are beginning to ossify, which means that if an x-ray was taken the skeleton would be visible. In the sixteenth week, several of the more complicated body systems are also beginning to function including the baby’s urinary and circulatory system. The baby’s heart pumps around 25 quarts of blood per day.

10. Second Trimester Week seventeen, the baby’s umbilical cord is not only lengthening but also becoming thicker and stronger for these last 23 weeks. The baby’s sense of hearing is also developing since the ears have fully formed and moved into their final position. The baby is beginning to form adipose or fat tissue. Fat is important in helping regulate the body’s temperature and metabolism. In the eighteenth week, a protective covering called myelin is beginning to form around the baby’s nerves. In week nineteen, the kidneys are making urine, and hair is beginning to sprout on the baby’s scalp. The parts of the brain that are responsible for the senses are specializing. If it is a baby girl, she already has 6 million eggs formed in her ovaries.

11. Second Trimester Week twenty the mother may begin to feel the baby’s first movements known as quickening. This is normally when an ultrasound can detect the sex of the baby. In week twenty one, the baby’s digestive tract is continuing to mature. If the baby is a girl, then her vagina has fully formed but will continue to develop until birth. Week twenty four is when the taste buds are developing, and the lungs are becoming more developed and complex. The branches of the main lung are beginning to form as well as special cells that will produce surfactant. Surfactant is necessary for the air sacs to inflate easily. In week twenty six, the nerves in the ears are developing and allowing the baby to respond more consistently to the sounds that they hear. If it is a boy, his testicles have begun their descent into his scrotum .

12. Third Trimester Around week 27 to 28, the baby can open and close their eyes. The baby has also developed a regular wake and sleep cycle. Up until this point, the baby’s brain has been relatively smooth, but beginning this week his brain will develop grooves and indentions along its surface. The amount of brain tissue also begins to increase during the 28th week. This causes the baby’s head to grow. In week thirty, the baby’s eyes are becoming more mature, and now he can tell the difference between light and dark. Babies at 30 weeks can even follow a light source with his eyes.

13. Third Trimester During the thirty second week, the baby’s toenails and fingernails have formed. The lungs are continuing to mature, but will not reach full maturity for several more weeks. The baby’s skeleton has completely formed, but his bones are very soft and pliable. In the thirty third week, the baby’s bones are all beginning to harden except for the skull. The skull needs to remain soft and pliable for delivery. Week thirty four, the central nervous system is continuing to mature, and the lungs are well developed. While most health care providers would ideally like the mother to carry up to 38 to 40 weeks, the baby has a good chance of surviving outside of the uterus at this time.

14. Third Trimester By the thirty fifth week, the vast majority of the baby's growth is complete. Their kidneys are completely developed, and the liver is beginning to process waste. In week thirty six, the fine downy hair, lanugo, that has covered the baby’s skin is beginning to disappear, along with the vernix caseosa. Vernix caseosa is the thick, creamy substance that has protected the baby’s skin while he has been submerged in amniotic fluid. Both of these are swallowed by the baby, along with additional amniotic fluid, which combine together to form meconium, the baby’s first bowel movement. By the end of week thirty seven, the baby is considered full term.

15. Third Trimester By the thirty eighth week, the vast majority of their organs are mature and fully functioning. The only exceptions are the brain and lungs. These two organs are at a point that they will function outside of the womb, but will also continue to mature during childhood. In the last two weeks, the baby’s bones have become hard, with the exception of his skull. The bones in the skull need to remain soft and pliable for delivery so that they can overlap as they pass through the birth canal.

16. References http://www.americanpregnancy.org/duringpregnancy/ http://www.mayoclinic.com/health/medical/IM04051