Conception occurs when two sexual reproduction cells–the spermatozoa and the ovum--meet to form a brand new cell. This cell divides, and divides again. The division continues until a cluster of cells is formed. This biological process is the beginning of life and the first stage of the formation of a new baby.
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A zygote is the first cluster of cells that becomes a baby. Zygotes contain chromosomes, bits of deoxyribonucleic acid or DNA surrounded and controlled by proteins. DNA, often referred to as “the building blocks of life,” contains all of the information the body needs to determine physical traits including sex, hair and eye color and other physical traits. The zygote has 46 chromosomes, 23 from each parent. Identical twins are formed if the zygote splits; fraternal twins occur when two different ovum are fertilized by two separate spermatozoa, a fact noted in "Biological Science Volume One: The Cell, Genetics and Development."
About three to five days after the formation of a zygote, the whole bundle of cells travels from the Fallopian tubes, where conception began, to the uterus. The zygote attaches to the uterine wall. During this phase, some of the cells form the placenta and amniotic sac, while others continue to form the baby.
At approximately the third week of pregnancy, the group of cells enters a stage known as the embryonic stage. The U.S. Department of Health and Human Services states that during the embryo stage, the child measures approximately 1/25 inch. The cluster of cells begins to take shape into a vague human form. The head takes shape. Tiny buds that will become the arms and legs start to form. The brain and spinal cord are starting to develop. By the 10th week of pregnancy, almost all major organs have started to form. At the end of the embryonic stage, the baby is about one inch long, according to the Department of Health and Human Services.
During this stage of pregnancy, starting at about the 10th week of pregnancy, the cluster of cells finally starts to look like a baby. It is no longer an embryo: It is a fetus. Week 13 marks the end of the first trimester of pregnancy. During the rest of the first trimester, the baby's external sex organs become visible, though they are still too small to be seen on an ultrasound, the American Pregnancy Association says. As the fetus grows, it begins to fill and expand the uterus. By Week 13, the child has eyelids and is about three inches in length.
Second Trimester Growth
Weeks 14 through 28 mark the second trimester of pregnancy. During this time, the baby has thin, transparent skin and is protected by a light dusting of hair called languno and a waxy coating known as yernix. Fingernails and eyelashes form, as do the fingerprints and footprints that are individual to the baby's being. Hair begins to form on the baby's head, and the child's startle reflex is developed. During this time, the baby's movements are large enough to be felt by the mother. Around 24 weeks, the child has a slim but possible chance of survival outside the uterus in the event of a premature birth. By the end of 28 weeks, the child is about one foot long.
Third Trimester to Birth
The third trimester is marked from Week 29 until birth. The child can open and close her eyes. The baby's bones and lungs form and mature. The child begins to form body fat. The thin down languo falls off. By Week 33, most babies start to turn to prepare for birth. By the end of the third trimester at birth, the baby is typically between 19 and 21 inches in length and weighs between six to 10 pounds, as noted by the American Pregnancy Association.