The organs developed in the first semester grow bigger, even though the seeds of development have been sown and "except for certain parts of the brain and lungs, all the cells the baby will ever have are there" by now (Bontragaer 2005, p.5). The third trimester, which takes place between 28 and 32 weeks, is a process of maturation: "In the first two trimesters, most fetuses are pretty much the same size at the same age…in the third trimester, babies start to grow at different rates" (Bontragaer 2005, p.6). Behaviors as well as anatomical structures are obvious, and third trimester fetuses swallow, breathe, sleep, wake, and even suck their thumbs (Bontragaer 2005, p.6). Thus the baby during the third trimester seems to be preparing itself to live in the outside world. "In the third trimester the fetus increases the surface area of its brains to astronomical proportions. In fact, so much surface area is made that it wouldn't all fit into our heads if our brains didn't fold up on themselves. So during the third trimester the brain goes from smooth to one that's full of folds and grooves. Plus, the baby begins to use his or her brains in a purposeful, rather than random, manner. The baby will start practicing things that he or she is going to need to be able to do after birth. The baby doesn't need to breathe while in the uterus, but will actually start to practice breathing exercises -- moving the chest, belly and diaphragm" (Bontragaer 2005, p.7).
When the time to give birth comes, the mother's body produces a hormone that causes the relaxation of the ligaments that hold together the pubic area of the birth canal, the meeting point of two adjacent pubic bones and the sacroiliac symphyses at the bottom of the spine. "The technical name for this loosening is 'subluxation.' In extreme cases, the pubic symphysis may open a centimeter or more, making it difficult or impossible for a woman to walk in the period just before birth. However the mean is about 4 mm. Even this can produce an adequate increase in the total area of the pelvic outlet" (Jordon 1996). This is necessary, given that by the end of the pregnancy, the baby's head is larger often than the birth canal, and "if there were no flexibility on either side, the baby could not be successfully delivered, and mother and baby would both usually die" (Jordon 1996).
The birth process takes place in four distinct phases. "Stage 1 consists of regular uterine contractions with cervix dilation. Full cervix dilation occurs at approximately 4 inches, Stage 2 generally takes from 15 to 50 minutes. During this time, uterine contractions strengthen and become more frequent. During this stage mother will feel the need to bear down and push. The baby goes through a series of passive movements - especially the head, which undergoes flexion, internal rotation, extension, external rotation, and crowning (the first sign of the baby's head)," which then cumulates in Stage 3, "the period immediately following birth to the expulsion of the placenta - generally taking 5 to 10 minutes. Should the placenta not easily come out, tugging or pulling should not be performed. Gentle uterine massage may be utilized to assist in the release. The placenta should always be examined to be sure no parts remain within the uterus. This can become detrimental to the mother causing hemorrhage and/or death" (Pregnancy, 1999, DC Doctor). Stage 4 cumulates in the expulsion of the placenta and afterbirth recovery.
Birth is a perilous and difficult process, even before conception, given the difficulty of ensuring the egg and sperm meet, attach to the uterine lining, and develop normally. Birth, even when it proceeds normally, is traumatic. Yet it is also remarkable, however ordinary it may seem: through sexual reproduction and rapid cell division and maturation, an entirely new human being is created. A unique individual with a unique genetic code is generated -- who may go on, him or herself, to create other unique human beings, after maturation.
Bailey, Regina. (2010). Sex chromosome abnormalities. About.com. Retrieved July 3, 2010 at http://biology.about.com/od/basicgenetics/a/aa110504a.htm
Bailey, Regina. (2010). Sexual reproduction. About.com. Retrieved July 3, 2010 at http://biology.about.com/od/basicgenetics/a/aa062708a.htm
Bontragaer, Jennifer. (2010). Fetal and embryo development. Pregnancy Today.
Retrieved July 3, 2010 at http://www.pregnancytoday.com/articles/fetal-development/embryo-development-6022/6/
Freudenrich, Craig. (2010). How sex works. How stuff works. Retrieved July 3, 2010 at http://health.howstuffworks.com/sexual-health/sexuality/human-reproduction2.htm
Hammonds, Christie. (2009, May). Telling a baby's sex. Retrieved July 3, 2010 at http://www.babycentre.co.uk/pregnancy/antenatalhealth/scans/sexbyultrasoundexpert/
Human reproduction and biology. (2004, March 3). IUPUI Department of Biology.
Retrieved July 3, 2010 at http://www.biology.iupui.edu/biocourses/n100/2k4ch39repronotes.html
Johnson, D.R. (2010). Retardation and neontony in human reproduction. University of Leeds.
Retrieved July 3, 2010 at http://www.leeds.ac.uk/chb/lectures/anthl_06.html
Jordon, J.K. (1996). Human Birth & Bipedalism. USC. Retrieved July 3, 2010 at http://weber.ucsd.edu/~dkjordan/resources/clarifications/HumanBirth.html
Miller, Ken. (1999). The peppered moth. Brown University. Retrieved July 3, 2010 at http://www.millerandlevine.com/km/evol/Moths/moths.html
Pregnancy: The birth process. (2010). DC Doctor. Retrieved July 3, 2010 at http://www.dcdoctor.com/pages/rightpages_wellnesscenter/pregnancy/birthingprocess.html
Sandel, Michael. (2007, April 8). Embryo ethics. The Boston Globe. Retrieved July 3, 2010 at http://www.boston.com/news/globe/ideas/articles/2007/04/08/embryo_ethics/
Scheibel, a. (2004). Embryological development of the human brain. New Horizons. Retrieved July 3, 2010 at http://www.newhorizons.org/neuro/scheibel.htm
Vorvick, Linda J. (2009, November 1). Fetal development. National Institute of Health.
Retrieved July 3, 2010 at http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm