Fetus

Fetus

Prenatal development of fetus;

1. Ovular period or germinal period 

Which last for first 2 weeks following ovulation.

2. Embryonic period

Begin 3rd week following ovulation and extends upto 10 week of gestation (8 week post conception).

3. Fetal period

8th week conception and ends of delivery.

Length of fetus

1. Crown-rump length

In earlier weeks, measurement taken from vertex to coccyx.

2. Crown- heel length

End of 20th week onward.

Measurement is taken from vertex to heel.

Age of Fetus

Gestational age is duration of pregnancy calculated from first day of last menstrual period (LMP).

Growth of Fetus

Characterized by cellular hyperplasia and hypertrophy.

Fetal growth factor; insulin/ insuline-like factor l (IGF-l).

Note - For postnatal growth, growth hormone is essential.

Average fetal weight; 2.5kg to 3.5 kg

Fetal Physiology

1. Nutrition

A. Absorption

In early post fertilization  period, nutrition stored in dertoplasm within cytoplasm.

Before the placenta forms, yolk sac provides nutrition and gas exchange between the mother and the developing embryo.

B. Histotrophic transfer

Developing uterine receive additional nutrition from its mother inform of uterine section known histotroph or uterine milk.

Following nidation and before establishment of utero placental circulation, also recieve nutrition from histotroph.

Total feto placental blood volume at term is estimated to be 125ml per kg body weight of the fetus.

2. Leucocytes and fetal defence

Leucocytes appear after 2 months of gestation.

Maternal immunoglobulin G (IgG) crosses the placenta from 12th week onwards.

Give passive immunity to fetus which increases with increase in gestation period.

Note; Cordocentesis, helpful in diagnosis of intrauterine infection.

IgA immunoglobulin excreted in milk, protect against enteric infection only after birth.

3. Urinary system

By the end of first trimester, nephrons become active and secrete urine.

Urine exerted into amniotic cavity by fetus form major component of amniotic fluid.

4. Haematotrophic

With the establishment of fetal circulation, nutrition is obtained by active and passive transfer from 3rd week onwards.

5. Fetal blood

Yolk sac is the first site of blood cell production during embryonic phase by 14th day.

By 10th week liver becomes major site.

Enlargement of early fetal liver due to erythropoietic function.

Gradually, RBC production sites extend to spleen and bone marrow and near term bone marrow become major site of red cell production. Life span of RBC; 80 days.

Note; Presence of Rh factor has been demonstrated in fetal blood from early 38 days after conception.

Hb of fetal blood 16.5- 18.5gm/dl

Cord blood level of iron, ferritin, vitamin B12, folic acid and are consistently higher than maternal blood.

FFN (fetal fibronectin test); if found positive increased risk of premature birth.

Near term, urine production rises to 650ml per day due to enlargement of kidney.

11th weeks after conception, fetus begining to make urine and release it into surrounding amniotic fluid. Fetus swallows some amniotic fluid and excreted.

Amniotic fluid also contains; Nutrients, Hormones, Antibodies.

Oligohyramnios (low amniotic fluid); associated with renal hypoplasia or obstructive uropathy.

Polyhydramnios; Excessive amniotic fluid

6. Skin 

At 16 weeks, Lanugo (soft hair) appear.

Sebaceous glands appear at 20th weeks,sweat gland appear later.

Vernix caseosa; Secretion of sebaceous gland mix with exfoliated epidermal cells.

7. Gastro-intestinal tract

At early,  10-12 week, fetus swollows amniotic fluid.

Meconium appears from 20th week and at term throughout gut upto rectum indicating the presence of intestinal peristalsis.

In intrauterine hypoxia, anal sphincter is relaxed meconium may voided into liquor amnii.

Composition of Meconium

Waste product of hepatic secretion.

Contain lanugo, hairs and epithelial cell from fetal skin hich are swallowed with liquor amnii.

Mucus, exfoliated intestinal epithelium and intestinal juice are added to content.

Greenish black color is due to bile pigments specially biliverdin.

8. Respiratory system

In early month, lung are solid.

At 28th week, alveoli expand and are lined by cuboidal epithelium.

At 24th week, lung surfactant appear.

Surfactant is secreted by type Il alveolar cells.

These surfactant substance lower the surface tension of lung fluid so that alveoli opened up easily when breathing starts following delivery.

Lecithin/ sphingomyelin (L/S) ratio test used for fetal lung maturity testing.

Lecithin: sphingomyelin (L:S) ration is 2:1 in liquid amnii signifies full maturity of fetal lung.

Fetal cortisol is natural trigger for surfactant synthesis.

Fetal growth restriction (FGR), prolonged rupture of membrane also delay surfactant maturation and synthesis.

Breathing movement identified by 11 weeks but are irregular until 20th week.

Frequency (30-70 per minute)

Hypoxia and maternal cigarette smoking reduces FBM (fetal breathing movement) while hyperglycemia increase fetal breathing movement.

Tracheobronchial tree is filled up with liquor amnii.

9. Fetal endocrinology

Fetal pancreas secretes insulin as early as 12th week and glucagon by 8 weeks.

Fetal adrenal show hypertrophy of reticular zone which is site of synthesis of cortisol, oestrol precursor, dehydroepiandrosterone sulfate (DHEAS).

Fetal thyroid starts synthesizing small amount of thyroxine by 11th week.