Full term pregnancy is considered 40 weeks and an infant delivered before the 37th week completed is considered premature. Pregnancy is counted from the first day of a your last period. This means that at conception, you are already considered two weeks pregnant.
The first, second, and third months are the first trimester of pregnancy.For more information on development, use the drop down menus above or the weeks links below.2 weeks after the first day of the last normal menstrual period – The sperm and egg join in the fallopian tube and fertilization has occurred. A single sperm penetrates the mother’s egg cell, and the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. The baby begins to form from this single fertilized egg, the cell starts to divide itself into 2 cells, then 4, 8 and so on rapidly. Forty-six chromosomes combine, which pre-determine all of a person’s physical characteristics. The fertilized egg is still rapidly dividing as it floats down from the fallopian tube and towards the uterus. The embryo first moves into the uterus at about 80 hours after ovulation. The implantation process begins about 3 days later.
3 weeks – Now the developing embryo is in the uterus, it searches for a nice place to implant. When one is found, the zygote burrows beneath the surface of the uterus. At this stage the fertilized egg is now called a blastocyst, it is a fluid filled cluster of 50 to 60 cells, still multiplying madly. Implantation of the blastocyst occurs at about day 5 to day 8 of embryo development. IVF embryos are usually transferred into the uterus around this stage. The developing embryo is only one-hundredth of an inch long, but is rapidly developing. It can reproduce itself through twinning at any time up to about 14 days after conception; this is how identical twins are formed.
4 weeks – At week four, you might begin to suspect you are pregnant since the embryo produces hormones which stop the mother’s menstrual cycle. The cluster of cells has attached to the uterus wall and has divided into two parts. The half attached to the uterine wall will become the placenta, the vessel filled support system that will nourish the developing life, and the other half will become the baby. The amniotic fluid that cushions the fetus begins to form. The cells of the embryo multiply and begin to take on specific functions. The backbone, spinal column, and nervous system are forming. The heart, kidneys, liver, and intestines will soon be taking shape. Nerve growth begins when a sheet of cells on the back of the embryo folds in the middle to form a tube, which will become the future spinal cord. At one end the tube enlarges to form the brain’s major sections.
5 weeks – On the 26th day after fertilization, the embryo’s tiny heart begins to beat. At this time the embryo is still size of a raisin. There is rapid growth, and the baby’s main external features begin to take form. Low on the sides of the head are two folds of tissue that will become the ears. Although not completely developed, all the major body organs and systems are formed. The neural tube enlarges into three parts, that will soon to develop to become a very complex brain. Also the placenta begins functioning, known as the chorionic villi and the umbilical cord, through which the baby will receive nourishment and oxygen. The spine and spinal cord is growing faster than the rest of the body at this stage and will give the appearance of a tail. This will disappear as the baby grows.
6 weeks – The embryo itself is about a 1/4 inch (6mm) long. Major organs, including the kidneys and liver, have begun to develop, and the neural tube, which connects the brain and spinal cord closes. The brain develops into five areas and some cranial nerves are visible. The arm and leg buds are visible. The physical sensations of pregnancy start — nausea, fatigue, sore breasts and frequent urination.
7 weeks –The embryo’s hands and feet are shaped like paddles, but the fingers are beginning to take shape. The embryonic tail has now almost disappeared. The pituitary gland is also forming and the fetus is beginning to grow muscle fibers. The heart has divided into the right and left chambers and is beating about 150 beats a minute which is about twice the rate of an adult. The baby’s facial features are visible, including a mouth and tongue. The eyes have a retina and lens. The major muscle system is developed, and the baby starts to practice moving. The baby has its own blood type and the blood cells are produced by the liver.
8 weeks – The unborn baby is now called a fetus. The fetus is protected by the amniotic sac and filled with fluid. Inside the child swims and moves gracefully. The fetus is now about 1/2 inch (12mm) long. The arms and legs have lengthened. During this time of development, the baby’s head appears much larger than the body because the brain is growing very rapidly. Brain waves can now be measured. The teeth and the palate are beginning to form and the larynx is developing. Through its parchment-thin skin, the baby’s veins are clearly visible. By the eighth week the ears begin to take shape.
9 weeks – Though the fetus is constantly moving, you won’t be able to actually feel fetal movement for several more weeks. All of the organs, muscles, and nerves are in place and beginning to function. The eyelids have fused shut and won’t open again until week 27. As the hands and feet develop fingers and toes, they lose their paddle-like look, and the touch pads on the fingers form.
10 weeks – The fetus weighs about 1/3 of an ounce. The heart is almost completely developed and very much resembles that of a newborn baby. An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child’s blood is oxygenated through the placenta. The wrists and ankles have formed and the fingers and toes are clearly visible. Genitals have begun to from, but it is too early to tell the sex of the fetus. By this week of the pregnancy the placenta has developed enough to support most of the critical job of producing hormones.
11 weeks – The vital organs – the liver, kidneys, intestines, brain and lungs are almost fully formed and beginning to function, while the the head is almost half the length of the entire body. The fetus is only about 2 inches (50mm) long and weighs less than a half ounce but it is busy moving and kicking.
12 weeks – Vocal chords are complete, and the baby can and does sometimes cry silently. The brain is fully formed, and the baby can also feel pain. The fetus may even suck his thumb. The eyelids now cover the eyes, and will remain shut until the seventh month to protect the delicate optical nerve fibers. The hair is on the head and the fingers and toes have developed soft nails. The kidneys are developed and begin to secrete urine.
Week 12 and the third month is the end of the first trimester.