Firstly a scientific overview is a bit different from the development often offered to mothers by their physician or similar sources. Bee states in her book The Developing Child that gestation from a scientific view, begins from the moment of conception when a single sperm joins with the ovum egg cell, which happens roughly midway through the menstrual cycle. This calculates pregnancy to have a gestation of approximately 38 weeks and is the timescale used in this overview, counting from the moment of actual conception, not the first day of the last menstrual period used for the common 40 week estimate.
Bee continues that the second major difference between medical and scientific approaches is the way that they are subdivided. Medically a pregnancy is assumed to be split into three ‘trimesters’, each an equal three months in length. Scientifically development looks very different, still having three periods of development but of very unequal length.
Thornton, in Understanding Human Development explains these; the zygotic stage spans from conception to implantation of the cells into the wall of the uterus at around 10 to 14 days after conception. The second embryonic stage spans from implantation to roughly 8 weeks gestation. The third and final period is the fetal stage and lasts from around 8 weeks until the birth, lasting roughly a huge 7 months of the pregnancy.
Germinal Stage of a Baby’s Development During Pregnancy.
Bee explains that conception usually occurs in one of the fallopian tubes. During the next week the fertilized ovum travels down the tube to the uterus. Thornton continues that during this time, between 12 and 36 hours after conception, the cell begins to divide to produce identical replicas, roughly doubling in number twice a day. Two to three days later the mass contains several dozen cells, and at around day four it becomes known as a blastocyst.
Thornton explains the development of the blastocyst is marked by subdivision, forming a hollow sphere with two layers of cells around it. The outer layer becomes the baby’s support structures; placenta, amniotic sac, umbilical cord etc. The inner layer becomes the future baby itself. The end of this stage happens at around two weeks. Bee explains this is when the blastocyst touches the wall of the uterus and breaks down at the point of contact, developing small tendrils which attach it securely, resulting in implantation.
Embryonic Stage of a Baby’s Development During Pregnancy.
The next six weeks consist of the embryonic stage, a period of rapid development which really shows how amazing the development of a baby is. Pinel in Biopsychology describes how at first all the replicated cells, known as stem cells, are identical and can potentially turn into any type of cell in the body. Pinel goes on to describe this process in five phases beginning with the development of the neural plate, which will soon form the spine and brain of the developing baby.
Next, in phase two, these cells are proliferated, multiplying in huge numbers, followed by the moving and combining of cells which gradually become more specialised to form particular parts of the developing baby. The fourth stage involves the growth and formation of connections between the cells forming the synaptic routes through which information from the body’s entire nervous system will be sent.
Finally these synapses die and rearrange, an essential part of development. Thornton provides an excellent example, explaining that arms develop with essentially a blob on the end. The blob develops ridges for the fingers, however without the systematic death of some cells between these ridges, the fingers would not develop, remaining webbed.
The Miracle of a Baby’s Development During Pregnancy.
McNabb (1997) describes in The Biology of Early Pregnancy that the first organ system to initiate development is the nervous system, beginning at only 18 days after fertilization! This begins when the mass intended to develop into the baby, develops into three layers forming the aforementioned neural plate.
The first layer will become skin, nails, teeth, ears, eyes and the nervous system. The second will become inner flesh and skin, the circulatory system, muscles, bones and other organs. The final layer will become the lungs, digestive and urinary systems and the glands. Thornton continues describing that once these three layers are established, a groove develops, gradually growing around to form a complete tube which will become the spinal cord and brain.
Developmental Milestones of the Developing Baby at Only Four Weeks Pregnancy Gestation
Bee summarises the amazing developments at only four weeks and up to eight weeks gestation. By approximately four weeks the placenta and umbilical cord are already fully developed in basic form. Alongside the support system, the baby itself is rapidly developing and by again four weeks, a heartbeat can be detected and the beginning of lungs and limbs are also present.
By the end of the eight week period all internal organs are present, skin, limbs bones are visible and fingernails are growing. The baby is now moving and can even respond to touch. Stoppard (1999) in the ‘new pregnancy and birth book’ estimates the size of this already considerably developed baby to be approximately only one inch by week eight. The first eight weeks of development are the most biologically amazing, rapid and also the most crucial. Essentially development has all been organised to a rudimentary degree and the seven month long, third period of development, consists mainly of these structures growing in detail and size.
Fetal Stage of a Baby’s Development During Pregnancy
By the end of the embryonic stage the nervous system only consists of a hollow tube out of which the both brain and spinal cord continue to develop during the fetal stage (Bee). Thornton notes that the baby seems in a hurry to develop fully before gaining weight, and by the 28th week can survive by itself if born prematurely. In fact even at around 22 weeks, barely past the half way point of the pregnancy, the baby is well enough developed that it may survive with medical care.
Bee further supports this incredible fact by indicating that the baby reaches birth length by about 20 weeks, but does not reach even half of birth weight until nearly three months later. The weight gain is left until after development, and according to Stoppard, from week 36 the baby gains an incredible one ounce every day for the next four weeks until its birthday!