Pregnancy is measured from the first day of a woman's last period, although the conception typically happened some 14 days after this. The first 12 weeks of pregnancy, or the first trimester, is the most critical for the growth and development of the new life.
Conception
New life begins when the egg and sperm join to form a zygote, or a fertilized egg. This typically happens in the fallopian tubes that connect the ovaries with the uterus. Right after this, the fertilized egg begins to divide. The first cell division is very slow, taking approximately 30 hours to complete, but after this, cell division occurs more and more rapidly. At this stage, all the new cells formed are identical.
Traveling Down the Fallopian Tube
While this group of cells is dividing rapidly, it is also traveling down the fallopian tube towards the uterus. This cell ball, or zygote, is not capable of moving, but the muscular contractions of the fallopian tube and the finger-like structures inside the tubes move the zygote toward the uterus. During this time, the zygote gets nourishment from inside the fallopian tube and is not dependent on the food that the mother is consuming. Dr. T.W. Saddler states in his book "Langman's Medical Embryology," published in 2006, that approximately 30 percent of the zygotes never make it this far and the woman never even finds out that she was pregnant. In some of these cases, the sperm and the ovum did not join correctly. In others, the cell division never started, or stopped after a while.
Implantation
Seven to nine days after fertilization, the zygote arrives in the uterus and connects to the uterine lining. From this moment, called implantation, the zygote is known as an embryo. Now, it is connected to mother's bloodstream. This is significant in many ways. First, everything that mother eats and drinks can now either nourish or hurt the very delicate embryo. Also, the hCG hormone is being released into the mother's bloodstream and some pregnancy tests can begin to detect the pregnancy. Some mothers may notice slight spotting around the time of the implantation and mistake it for a period.
Placenta Develops
Structures that feed and protect the organism begin to develop around this time. These include yolk sac, placenta and umbilical cord. By the end of the second week, a protective membrane is formed, which surrounds the amnion, or the prenatal organism. The placenta develops very shortly. It has a challenging role. It has to keep the bloodstreams of the mother and the embryo separate. However, it also needs to allow the embryo to get food and oxygen from the mother. Additionally, the placenta helps carry the embryo's wastes away. The placenta is connected to the embryo via the umbilical cord.
The Period of the Embryo
The period of the embryo lasts from implantation through the first eight weeks of pregnancy. During this time, development is very rapid. Because all the vital body parts and inner organs form during this time, the embryo is very vulnerable. For example, at only three weeks, the brain begins to form. At eleven weeks, the embryo becomes a boy or a girl. By the end of the first trimester, the embryo will have fingers and toes and can even move. Now, it is called a fetus.
Early Pregnancy Symptoms
While so much is happening inside a new mother, not much may show on the outside. Among the first pregnancy symptoms are typically swollen or tender breasts, while some women experience nausea and vomiting, which typically stops after 12 weeks of pregnancy. Mothers-to-be may feel tired or experience mood swings.
References
- National Library of Medicine: Fetal development
- "Langman's Medical Embryology"; T.W. Saddler, M.D.; 2006.
- Children's Hospital of Pittsburg:The First Trimester
