Pre-embryonic development

The fertilized ovum undergoes mitotic division resulting in a zygote then a morula. Within 4 days it becomes a blastocyst which implants into the uterine endometrium.

Embryonic development

Three germ layers – the ectoderm, mesoderm and endoderm – form the main tissues and organs in the developing embryo. By 4 weeks the embryo has a head, a trunk and tiny limb buds. Blood is also circulating around embryonic body at this stage. By 4–8 weeks, the embryo measures 2.5 cm and has recognizable facial features and major sensory organs are developing such as eyes, ears, nose, lips and tongue. The cardiovascular system, kidneys and liver are functional and the lungs are developing. Skeletal growth is cartilaginous and the limbs begin to lengthen.

Fetal development

8 weeks. The embryo becomes the fetus.

9–12 weeks. The eyelids close, the oral palate fuses, urine is produced and genitals are established. Red blood cells are produced in the liver. The fetus measures approximately 6.5 cm and weighs around 18 g.

13–20 weeks. Ossification of the skeleton begins and limbs reach their final proportions. Head hair is visible and lanugo is present over the body as is vernix. The liver and pancreas secrete enzymes and the fetus is capable of sucking. The fetus measures around 15 cm at this stage.

21–28 weeks. Lungs mature with alveoli visible. Fat deposits increase and lanugo begins to disappear. There is rapid brain growth at this stage.

28–32 weeks. Skin is pink but lacking in subcutaneous fat, but brown fat deposits increase. The main organs are functional and cerebral development increases as the brain grows. Length is approximately 38 cm and weight around 1800 g.

34–37 weeks.