Foetal Period

The foetal period lasts from the eighth week of development up until birth. It primarily involves the growth of the foetus, including the differentiation and maturation of the tissues and organs. This period transforms the embryo from a tiny entity resembling a human being into a fully developed baby which is ready to be born into the world.

Week 9 – 12

During this four week period the body and limbs grow rapidly to catch up with the disproportional head. By the end of this interval the upper limbs have grown to their final relative lengths, but the lower limbs are still slightly proportionally shorter. By Week 11 the intestines have retracted back into the body from the umbilical cord. At the beginning of this period the liver is the main site of red blood cell production (erythropoiesis) but by the end of this period that function has moved over to the spleen. The external genitalia begin to form during this time and are distinguishable by Week 12, allowing gender identification. Urine production also begins during this time which is excreted into the amniotic fluid, which the foetus swallows to reabsorb some of the fluid with the waste products being transferred to the mother through the placenta.

Week 9

An 11-week old foetus

Week 13 – 16

During this period the lower limbs reach their final relative lengths and growth of the body accelerates with the foetus doubling its crown-rump length (CRL) over these four weeks. The limbs begin to move and become co-ordinated, which is only visible via ultrasonography. The eyes have slight movements and relocate to face forwards, hair patterning is determined, and in females the ovaries are fully differentiated and contain follicles for the next generation of life.

Week 15

A 15-week old foetus

Week 17 – 20

Growth slows during these weeks but is still relatively fast. Foetal movements are felt by the mother, and the skin is covered in a waxy material called the vernix caseosa which is composed of dead epidermal cells and fatty secretions from the skin. Its function is to protect the foetus from any abrasions or chapping in the amniotic fluid. The foetus is also covered in fine downy hair called lanugo. Head hair and eyebrows begin to appear, and brown fat forms as a source of heat production. In females the uterus has formed by Week 18 and in males the testes have begun to descend.

Week 19

A 19-week old foetus

Week 21 – 25

During this period the foetus puts on substantial weight and rapid eye movements begin. Fingernails are present by Week 24, and also during this week the type II alveolar cells have begun to secrete surfactant which prevents collapse of the developing alveoli (see organ development page). A foetus born prematurely during this period may survive if given intensive care but may perish soon after due to the immature respiratory system.

Week 24

A 24-week old foetus

Week 26 – 29

During this period the central nervous system has matured enough to control body temperature and breathing movements. Subcutaneous fat and toenails are present and the eyelids open by Week 26. Erythropoiesis shifts from the spleen to bone marrow by Week 28. If the foetus is born prematurely during this period it will probably survive as the lungs have developed enough to allow adequate gas exchange.

Week 26

A 26-week old foetus

Week 30 – 34

This period is relatively uneventful, with only the establishment of the pupillary light reflex in the eyes. The skin turns pink and smooth and the foetus continues to grow and put on weight.

Week 31

A 31-week old foetus

Week 35 – 38

During this final period before birth the nervous system is further matured to the point where the foetus can analyse and interpret sensory information and illicit an appropriate motor response, particularly to light. In males the testes have fully descended. Growth slows but continues, and in the last few days the foetus puts on 14g of fat a day. The average CRL of a foetus at birth is 36cm and the average weight is 3400g.
The fully developed baby is then born at the end of Week 38, or Week 40 if using gestational age, in a process called partuition. The baby takes its first breath as soon as it enters the outside world, triggered by the cold shock and immense pressure exerted on it during labour.

Week 36

A 36-week old baby (premature birth)