The fifth week of pregnancy is the time of the first missed period when most women are only just beginning to think they may be pregnant. Yet already the baby’s nervous system is developing, and the foundations for its major organs are in place. At this stage, the embryo is around 2mm long and its heart has started to beat.
As the ectoderm develops, a groove forms and the layer of cells folds to form a hollow tube called the neural tube. This will become the baby’s brain and spinal cord.
At the same time, the heart is forming a simple tube-like structure. The baby already has some of its own blood vessels and blood begins to circulate. A string of these blood vessels connects the baby and mother and will become the umbilical cord.
By the time you are six to seven weeks pregnant, there is a large bulge where the heart is and a bump at the head end of the neural tube. This bump will become the brain and head. The embryo is curved and has a tail — it looks a bit like a small tadpole. The heart can sometimes be seen beating on a vaginal ultrasound scan at this stage.
The developing arms and legs become visible as small swellings (limb buds). Little dimples on the side of the head will become the ears, and there are thickenings where the eyes will be. By now the embryo is covered with a thin layer of see-through skin.
By seven weeks, the embryo has grown to about 10mm long from head to bottom. This measurement is called the ‘crown-rump length’. The brain is growing rapidly and this results in the head growing faster than the rest of the body. The embryo has a large forehead, and the eyes and ears continue to develop. The inner ear starts to develop, but the outer ear on the side of the head won’t appear for a couple more weeks.
The limb buds start to form cartilage, which will develop into the bones of the legs and arms. The arm buds get longer and the ends flatten out — these will become the hands. Nerve cells continue to multiply and develop as the nervous system (the brain and spinal cord) starts to take shape.
By the time you’re eight weeks pregnant, the baby is called a fetus, which means ‘offspring’. The legs are lengthening and forming cartilage. The different parts of the leg aren’t properly distinct yet — it will be a bit longer before the knees, ankles, thighs, and toes develop. The fetus is still inside its amniotic sac, and the placenta is continuing to develop, forming structures called chorionic villi that help attach the placenta to the wall of the womb. At this stage, the fetus still gets its nourishment from the yolk sac.