During the first trimester of pregnancy, a baby goes through many stages of development, starting as a fertilized egg after conception and then developing into a blastocyst, an embryo and then a fetus.
The growth and development of a baby up to eight weeks of pregnancy are described in more detail below.
Senior doctor reviewing woman's pregnancy scans - Image Copyright: Rocketclips, Inc.
Weeks 0 to 4
During the first two weeks of the time that is referred to as pregnancy, a woman is not actually yet pregnant.
Conception generally takes place approximately two weeks after the first day of a woman’s last period and doctors include the time from that first day as part of the pregnancy.
During those first two weeks, a woman’s body prepares for ovulation and an egg is released at around day 14 of the menstrual cycle. The egg is then swept into the funnel-like end of one of the fallopian tubes, the usual site for fertilization.
If the egg is fertilized by a sperm, then approximately three weeks after the first day of a woman’s last period, the fertilized egg (zygote) is moved slowly along the fallopian tube towards the uterus, by tiny cilia that line the tube.
The cells of the zygote divide repeatedly as it moves along the fallopian tube and the zygote takes between 3 and 5 days to enter the uterus.
Once inside the uterus, cell division continues and eventually forms a hollow ball of cells referred to as a blastocyst. This blastocyst burrows into the uterus wall (implantation) and develops into an embryo.
In weeks 4 to 5, the embryo starts to grow and develop, with the inner cells starting to form two and then three layers. An inner layer referred to as the endoderm is formed, which goes on to become the baby’s breathing and digestive system.
A middle layer, referred to as the mesoderm becomes the bones, muscles, heart and blood vessels and an outer layer called the ectoderm develops into the brain and nervous system. Throughout these early weeks, a tiny yolk sac supplies the embryo with nourishment until the placenta fully forms a few weeks later and takes over the role of providing nourishment and oxygen.
It is during this week of pregnancy that a woman misses her period. At this point, the embryo is about 2mm long and is already developing a nervous system. As the ectoderm develops, a layer of cells start to fold to form the neural tube that later becomes the brain and spinal cord.
Another tube-like structure that will become the heart also begins to form and some blood vessels are already present. Some of these blood vessels link the baby to the mother and eventually form the umbilical cord.
During the sixth week, the baby’s growth is rapid, with the neural tube starting to close and the heart starting to pump blood. At this stage, a heartbeat can sometimes be seen on an ultrasound scan.
The basic features of the face start to become apparent including canals that will form the inner ear and arches that will make up part of the jaw.
There are also thickened areas where the eyes will form. The body starts to become curved shaped and small limb buds become visible that will develop into the legs and arms.
A bump forms at the end of the neural tube which will develop into the brain and head.
By week 7, the embryo measures around 10mm and the brain, head and face start to develop rapidly. Nerve cells undergo continued development as the brain and spinal cord begin to take shape. The forehead is large and the lenses of the eyes start to form.
The inner ear also starts to develop and the arm buds lengthen and become paddle shaped. These will eventually develop into hands.
By week 8, the baby is referred to as a fetus. The legs form cartilage and increase in length, although leg parts such as the knees, thigh and toes are not yet distinct. Fingers have started to form, the ears have started to become shell shaped and the eyes are visible.
The fetus is still contained within the amniotic sac and receives nutrients from the yolk sac, but the placenta continues to develop and forms structures that will enable it to become attached to the womb. The fetus may now be around 11 to 14mm in length.
The mother’s body
A pregnant woman will probably not notice any signs of pregnancy during the first four weeks. The first sign a woman usually notices is that she has missed her period. By the eighth week of pregnancy, a woman will have missed another period, although some women experience a small amount of bleeding during the early weeks, referred to as implantation bleeding.
By the seventh or eighth week, the womb is approximately the size of a lemon, the breasts may be tender and there may be a need to urinate more frequently. Women may also feel more tired than usual and start to experience feelings of nausea or morning sickness.