Upon fertilization, the chromosomes of both the egg and sperm come together. At that point, the sex of the fertilized egg will already have been determined. If it is a boy, that means the sperm that fertilized the egg has a Y chromosome; if it is female, then the sperm has two X chromosome. In fact, the entire genetic code is present in the single-cell organism at this stage. When an egg becomes fertilized, it is then called a “zygote.”
In order to keep the remaining sperm out (generally, about 250 additional sperm will have traveled to the egg’s vicinity), the fertilized egg closes its outermost membrane immediately. Within minutes of conception, the cells begins rapidly dividing and multiplying and then the zygote transforms into a blastocyst. The cells that are being produced will aid in the baby’s development, while others are responsible for making up the umbilical cord, amniotic sac, and placenta.
Implantation occurs when the blastocyst fixes itself to endometrium of the uterus. It can take as few as three or four days, but may also take up to ten days. An exchange of hormones takes place to aid the blastocyst implant. Once it has become implanted the developing, fertilized egg is referred to as “embryo.” Pregnancy is counted from the first day of your last menstrual period (LMP). From that point, most pregnancies last about 40 weeks. 1
Within the very first week of pregnancy, the one-cell zygote transforms into millions of cells. There are three different cell groups that begin taking action at that point. The mesoderm forms muscles, kidneys, reproductive organs, circulatory system, and skeleton; the ectoderm forms the senses, skin, and nervous system; and the endoderm forms the respiratory and gastrointestinal systems. By week two, hormones emanate from the embryo, causing the mother’s menstrual cycle to halt for pregnancy. By the third week, the heartbeat starts. During week four, the brain develops, as well as the eyes, ears, and a mouth opening. The embryo begins pumping its own blood, and arms and legs begin to develop. It is also during this time that the umbilical cord is developed and the placenta begins functioning. The placenta is an organ that supplies developing babies with oxygen and nutrients, and also removes waste as needed. Development continues to take place, and it is usually by week eight that most people begin to refer to the developing life as a “fetus.” It is also during that point that joints form, and the heart beats about 60 times per minute.2 [avafootnote]
- American Pregnancy Association. (2015 July). “5 Weeks Pregnant: The Fifth Week of Pregnancy.” Retrieved October 31, 2015, from http://americanpregnancy.org/week-by-week/5-weeks-pregnant/ ↩
- Hartford, D. (2012). “The Early Prenatal Development of a Person.” VSM Resources. Retrieved October 23, 2015, from http://www.vsmresources.com/english/christian-life-issues/develop/ ↩