Human Fetal Period

Monthly Changes

The growth of the head during fetal period is slow compared to the rest of body. At the beginning of 3rd month, the head constitutes approximately ½ of crown – rump length (CRL). By the beginning of 5th month, the size of the head is about 1/3rd of crown – heel length (CHL). At birth is approximately 1/4th of the crown heel length..

Third Month (9-12 wks)

Photo by euthman. 9th week of development

Changes are observed in:

1. Face:

Face becomes more human looking and broad.

2. Head

Head is disproportionately large and makes up nearly half of the fetus’ size.

3. Eyes

Eyes are directed ventrally. The eyelids are fused.

4. Ears

Ears rise to their definitive position on the side of head

5. Limbs

Limbs reach their relative length

6. Primary Ossification Centers

Primary ossification centers are present in the long bones and skull by the 12th week.

7. Preumblical Herniation

During the 6th week, the intestinal loops cause a large swelling (physiological herniation) in the umbilical cord. By the 12th week, loops withdraw into the abdominal cavity.

8. Erythropoiesis

At 9th weeks, the liver is the major site for erythropoiesis. After 12th week, the activity is reduced in liver and begins in spleen.

9. External Genitalia

External genitalia  appear to be well differentiated.

10. Muscular yActivity

Muscular activity can be evoked in aborted fetuses.

4th And 5th Months

Following changes are observed during the fourth and fifth weeks:

1. Length

The fetus rapidly grows in length and at the end of first half of intrauterine life, its crown rump length is approximately 150 mms, about ½ the total length of  newborn.

2. Weight

The weight remains less than 0.5 kg.

3. Hair

Fine downy hairs (known as lanugo hair ) can be seen. Head and eye brow hairs become visible.

4. Movements

Fetal movements can be observed (which are quickening with time)

Mother may feel a fluttering in the lower abdomen

5. Skin

17 week fetus has thin skin with little connective tissue.

6th And 7th Months

1. Weight

During the 2nd half of intrauterine life, weight of the fetus increases. During last two and a half months 50% of the full term weight (approximately 3.2 kgs) is added.

2. Skin

During the 6th month, the skin of the fetus appears wrinkled and has reddish colour due to  the absence of underlying connective tissue.

3. Respiratory and Nervous Systems

The respiratory and the central nervous system have not differentiated completely and coordination between the two systems is not yet well developed.

4. Length

By six and a half to seven months, fetus has developed a length of about 250 mm and has a weight of approximately 1.1 kg.  The infant has a 90% chance of surviving, if it is born at this time.

5. Movements

Respiratory movements and sucking movements can be observed around this period of time.

6. Hearing

Some sounds can be heard by the fetus as well.

Ninth Month (33-36 Wks)

Skin

During ninth month the dull redness of the skin fades and the wrinkles smooth out. The lanugo hair begin to disappear

Body And Limbs

The body become rounded and the body fat increases

Vernix Caseosa

By the end of intrauterine life, the skin is covered by a whitish fatty substance

Skull

Skull has the largest circumference of all parts of the body, at the end of ninth month.

At the time of birth:

1. The weight of a normal fetus is around 3 to 3.4 kgs
2. Crown rump length is about 360 mms
3. Crown heel length is about 500 mms.

Testes are in the scrotum in the males at the end of 9th month.

Time Of Birth

The date of birth can be indicated as 266 days, or 38 weeks, after fertilization. The sperm present in the reproductive tract up to 6 days prior to ovulation can survive to fertilize oocyte. The oocyte is usually fertilized within 12 hours of ovulation. This is why most pregnancies occur when sexual intercourse occurs within a 6-day period ending on the day of ovulation. The date of birth is calculated as 280 days or 40 weeks from the first day of last menstrual period. This method is fairly accurate as most fetuses are born within 10 to 14 days of the calculated delivery date. The babies are called premature, if they are born much earlier. If born later, they are called postmature.

Measurements And Characteristics Of Fetuses

Various measurements and external characteristics are useful for estimating the fetal age. Until the end of first trimester, crown rump length  is the method of choice. In the second and third trimesters, several structures can be identified and measured ultrasonographically. Gestational sac appear on transabdominal sonography at 5th week (embryo at 7th week) and approx a week earlier by transvaginal ultrasound (TVS) (embryo at 6th week)

Ultrasonography Measurements

Ultrasonography measurements include:

1. Biparietal diameter (BPD)
2. Head circumference
3. Abdominal circumference
4. Femur length
5. Foot length
6. Fetal weight

Factors Influencing Fetal Growth

Following factors stimulate fetal growth:

1. Glucose
2. Amino acids
3. Insulin (secreted by fetal pancreas)
4. Insulin like growth factors
5. Human growth hormone

Intrauterine Growth Restriction:

Intrauterine growth restriction or IUGR is a term applied to infants who are at or below the 10th percentile for their expected birth weight at a given gestational age. They are small for dates or small for gestational age. The incidence is 1 in 10 babies

Factors Causing IUGR:

Different factors causing intrauterine growth restriction include:

1. Cigarette smoking
2. Alcohol consumption
3. Maternal malnutrition
4. Faulty food habits
5. Multiple pregnancies
6. Drug addiction
7. Impaired uteroplacental and fetoplacental blood flow (due to small chorionic vessels, severe hypotension, placental dysfunction or defect and renal diseases)
8. Genetic factors and growth retardation (such as recessive genes, structural and numerical chromosomal aberrations)