Development of a fetus – The term developing ovum is used for the first seven to ten days after conception, i.e., until the implantation occurs. It is called an ‘embryo’ from one week to the end of the second month, and later it is called ‘fetus’. It becomes an infant when it is completely born.
Gestational age in development of a fetus can be determined from maturation of chorionic villi; foot length and ossification centers.
End of first month : Length one cm; weight two- and-half g. The eyes are seen as two dark spots, and the mouth as a cleft. Nucleated red cells begin to form in placenta.
End of second month: Length four cm; weight ten g. The hands and feet are webbed. The placenta begins to form. The anus is seen as a dark spot. First ossification center in a fetus appears in clavicle (4 to 5 weeks), followed by maxilla (6 weeks).
Development of a fetus
End of third month: Length nine cm; weight thirty g. The eyes are closed and the pupillary membrane appears. Nails appear and the neck is formed
End of fourth month: Length 16 cm; weight 120 g. Sex can be recognised. Lanugo hair is seen on the body. Convolutions begin to develop in brain. Meconium is found in the duodenum.
End of fifth month: Length 25 cm; weight 400 g. Nails are distinct and soft. Light hair appears on head. Skin is covered with vernix caseosa. Meconium is seen at the beginning of the large intestine.
End of sixth month: Length 30 cm; weight 700 g. Eyebrows and eyelashes appear. Skin is red and wrinkled and subcutaneous fat begins to be deposited. Vernix caseosa is present. Meconium in transverse colon. The testes are seen close to the kidneys.
End of seventh month: Length 35 cm; crown- rump length 23 cm; foot length 8 cm; weight 900 to 1200 g. Nails are thick. Eyelids open and pupillary membrane disappears. Skin is dusky-red, thick and fibrous. Meconium is found in the entire large intestine. Testes are found at external inguinal ring. Gallbladder contains bile and caecum is seen in the right iliac fossa. Ossification centre is present in the talus.
End of eighth month: Length 40 cm; weight one-and-half to two kg. Nails reach the tips of fingers. Scalp hair is thicker, 1.5 cm. in length. Skin is not wrinkled. Left testis is present in the scrotum. Placenta weighs 500 g.
End of ninth month: Length 45 cm; weight 2.2 to three kg. Scalp hair is dark and 4 cm. long. Meconium is seen at the end of large intestine. Scrotum is wrinkled and contains both testes. Placenta weighs 500 g. Ossification centres are usually present in the lower end of the femur.
End of tenth month: (fullterm child): Length 48 to 52 cm; crown-rump length 28 to 32 cm; weight 2.5 to 5 kg; average about 3.4 kg. The length is much less variable than the weight. The male infant weighs about 100 g. more than the female. The circumference of the head is 33 to 38 cm. Six fontanels are usually present in the neonatal skull. The anterior fontanel (bregma) is located at the junction of sagittal and coronal sutures.
The posterior fontanel (occipital) lies at the junction of sagittal and lambdoid sutures. At each sphenoparietal junction, a lateral fontanel is present. The posterolateral fontanels (mastoid) are located at the mastoid-occipital junctions. Anterior fontanel is 4 x 2.5 cm. At full term the head of a child is nearly one-fourth of the whole length of the body. The surface of the brain shows convolutions, and the grey matter begins to form. The scalp hair is dark, 3 to 5 cm long. The face is not wrinkled.
Lanugo is absent except on the shoulders. The skin is pale and covered with vernix caseosa. The nails project beyond the end of fingers but reach only the tip of the toes. The cartilages have formed in the nose and ears. The testes are present in the scrotum; vulva is closed and labia minora are covered by fully developed labia majora.
The rectum contains dark brownish, green or black meconium. The umbilicus is situated midway between pubis and xiphoid cartilage. The umbilical cord is 50 to 55 cm. long, and one cm. thick. The centre of ossification is found in the lower end of femur and sometimes in the cuboid and in the upper end of the tibia. The placenta is 22 cm. in diameter, one- and-half cm. thick at the centre, and weighs about 500 gm.
Ossification Centres
(1) Sternum: It is placed flat on a wooden board and cut with the cartilage knife in its long axis in midline.
(2) Lower end of femur and upper end of tibia: The leg is flexed against the thigh and a transverse or vertical incision is made into the knee joint. The patella is removed. The end of the femur is pushed forward through the wound, and a number of parallel cross-sections are made through the epiphysis starting from its articular surface and continuing until the largest part of ossific centre is reached. The centre is seen as a brownish-red nucleus surrounded by bluish-white cartilage.
Further sections are made through plain cartilage above it until the diaphyseal centre is reached. The centre appears about the 36th week. Its diameter is about four to five mm. at 37 to 38 weeks, and 6 to 8 mm. at full term. The upper end of tibia is similarly examined. In 80 percent of full term infants, a centre is present in the upper end of tibia, but in other cases it appears after birth.
(3) Bones of the foot: The foot is grasped in the left hand behind the heel, the toes pointing towards the dissector. An incision is made between the interspace of third and fourth toes with a long knife, backwards through the sole of the foot and heel. If centre in calcaneum and talus are not exposed, thin slices of cartilage of these bones should be cut until the presence or absence has been shown. Centre in the calcaneum appears at the end of the fifth and in talus at the end of seventh month of intrauterine life. A centre may be present in cuboid at birth, or it may appear shortly afterwards.
RULE OF HAASE (1895)
This is a rough method of calculating the age of the foetus. The length of the foetus is measured from the crown to the heel in centimeters. During the first five months of pregnancy the square root of the length gives the approximate age of the foetus in months, e.g. a foetus of 16 cm. is four months. During the last five months, the length in cm. divided by five gives the age in months, e.g., foetus of 35 cm. is 7 months.
After birth, the length of an infant is 50 cm; 60 cm. at the end of six months; 68 cm. at the end of first year, and 100 cm. (double its length at birth) at the end of fourth year and tripled by 13 years. From age two to the beginning of adolescence, the average child will grow 5 cm. per year. Birth weight doubles by four to five months of age and triples by about one year and quadruples at 24 months. After age two, average weight gain until adolescence is 2 to 3 kg annually.
Head circumference increases by about twelve cm. in the first year of life. 90% of adult head size occurs by end of second year. The brain at birth is about 80% of its adult weight, and 90% by age six. In infants and children, height and weight may be compared with standard tables.
