The Third Trimester: Fetal Development




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The fetal development will continue to progress throughout the third trimester of your pregnancy.  During this stage of the pregnancy, your baby will be gaining more weight, their eyes will be opening and closing, and your body will begin preparing itself for the demands of labor and delivery.  Your pregnancy is drawing near an end, and you may be quickly approaching a point where you are absolutely tired of being pregnant.  Your thoughts may be preoccupied with having the opportunity to come face to face with your new baby, but you will quickly discover that there is still a lot of activity in your uterus.  It will help to gain an understanding of the various stages of development that your baby is undergoing as your delivery date draws closer.  The following is a calendar of the last weeks that comprise the third and final trimester of your pregnancy:

Week 28:  During this week of your pregnancy, which is approximately 26 weeks after conception, the eyelashes have formed and your baby’s eyelids are partially open.  The baby is continuing to gain weight, and this gain is serving to help smooth out some of the wrinkles that are present on the skin.  Your baby may be more than 10 inches (250 millimeters) long from crown to rump and weigh nearly 2 ¼ pounds at this stage of the pregnancy.  Generally healthy babies that are born at this stage of the pregnancy have a 90% survival rate without neurological or physical impairment – and the survival rate odds get better with each successive week of the pregnancy.

Week 29:  During this week of your pregnancy, which is approximately 27 weeks after conception, the baby’s skeletal system is fully developed, however, the bones themselves are still quite pliable and soft.  This stage of the pregnancy marks the time when the baby will start to create stores of minerals, such as iron, phosphorus, and calcium.

Week 30:  During this week of your pregnancy, which is approximately 28 weeks after conception, the baby may have a full head of hair, and the eyes are wide open.  Your baby may be more than 10 ½ inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds at this stage of the pregnancy.

Week 31:  During this week of your pregnancy, which is approximately 29 weeks after conception, the sexual development of the baby is continuing to progress.  If your baby is female, her clitoris is quite prominent, and if your baby is male, his testicles are traveling into the scrotum through the groin.  

Week 32:  During this week of your pregnancy, which is approximately 30 weeks after conception, the baby is starting to practice breathing, even though the lungs are not quite fully developed.  The baby’s body is starting to absorb vital minerals, such as calcium and iron, and toenails have become visible on its little feet.  The lanugo, which is the layer of soft thin hair that has been covering the baby’s body for several months, has started to fall off by this stage of the pregnancy.   You may begin to notice that the force behind the pokes and kicks that you have been feeling thus far are starting to become stronger.  Your baby may be more than 11 inches (280 millimeters) long from crown to rump and weigh nearly 3 ¾ pounds at this stage of the pregnancy.

Week 33:  During this week of your pregnancy, which is approximately 31 weeks after conception, the pupils of the baby’s eyes have developed to the point that they can dilate and constrict, effectively detecting light that enters the eyes.

Week 34:  During this week of your pregnancy, which is approximately 32 weeks after conception, the fingernails of the baby’s small hands has reached the tips of the fingers.  Your baby may be approximately 12 inches (300 millimeters) long from crown to rump and weigh nearly 4 ½ pounds at this stage of the pregnancy.

Week 35:  During this week of your pregnancy, which is approximately 33 weeks after conception, the vernix caseosa, which is the white coating that protects the skin of the baby, has started to thicken.  The overall shape of the baby’s body is becoming noticeably rounder.

Week 36:  During this week of your pregnancy, which is approximately 34 weeks after conception, the rate at which the baby is gaining weight has started to increase.  The baby will gain approximately ½ pound (227 grams) per week for the remaining weeks of your pregnancy.  The growing size of the baby may tend to limit the pokes and kicks that you were feeling prior to now, however, stretching, wiggling, and rolling may become noticeable.  You may want to try to pay attention to the activity levels of the baby at this stage of your pregnancy.  Discuss the expected hourly activity level of your baby with your health care provider, and feel free to contact them should you start to notice a decrease in the level that is anticipated.

Week 37:  During this week of your pregnancy, which is approximately 35 weeks after conception, you have reached the point where your baby will be considered full term.  The baby’s internal organs are fully developed, and they can be expected to function properly on their own.  In preparation for the upcoming birth, the baby may start to shift into a position where the head is pointed downward.

Week 38:  During this week of your pregnancy, which is approximately 36 weeks after conception, your baby’s toenails have reached the tips of the toes, and he or she is developing a firm grasp.  With the passing of each day, the nervous system and the brain of the baby are functioning more efficiently, and this process of development will continue through childhood and adolescence.  Your baby may be approximately 13 ½ inches (340 millimeters) long from crown to rump and weigh nearly 6 ½ pounds at this stage of the pregnancy.

Week 39:  During this week of your pregnancy, which is approximately 37 weeks after conception, the chest of the baby is starting to become more prominent.  In regards to sexual development, for the girls, the labia majora is well developed, and for boys, the testicles are continuing their decent into the scrotum.  The fat layer located below the skin has developed to the point where it can maintain body temperature with just a slight level of assistance from mom.  The baby has been receiving a continual level of antibodies from the placenta, which will serve to offer protection from infections during the first six months of life.  Should you choose to nurse your baby, the milk that you produce will provide even more antibodies.

Week 40:  During this week of your pregnancy you have reached your due date, which is approximately 38 weeks after conception.  Your baby may be approximately 14 inches (360 millimeters) long from crown to rump and weigh nearly 7 ½ pounds.  Please keep in mind, however, that this is an approximation, and that completely healthy babies come in all sizes!  You should not be alarmed if the specific due date has arrived and passed with no signs of active labor.  It is completely normal for a baby to be born a week or two late!  It is also just as normal for a baby to be born early.  Lastly, congratulations!




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