Fetal period

•        The period that extends from 9th week to the end of IUL.

•        Characterized by:

•        Maturation of tissues & organs

•        Rapid growth of the body

•        Few structural birth defects may be induced during this period.

•        It’s the time when prenatal screening techniques detect the birth defects.

•        CNS is vulnerable to insults that results in post natal behavioral disturbances, learning disabilities & lower intelligence.

Measurements of the fetus

•        Length of the fetus is indicated as:

ü CRL – crown rump length (sitting height)

ü CHL - crown heel length (standing height)

ü Biparietal diameter (BPD)

ü Head & abdominal circumference

ü Femur length

•        Expressed in cm & correlated with the age of the fetus in weeks or months.

 

Length & weight of fetus

•        Growth in length is striking during 3^rd – 5^th months.

•        Increase in weight is striking during the last 2 months of gestation.

Length of pregnancy

•        Length of pregnancy is considered to be 280 days or 40 weeks after the onset of last normal menstrual period (LNMP) or

•        260 days or 38 weeks after fertilization.

•        Most fetuses are born within 10-14 days of the calculated delivery date.

•        If born earlier – premature & born later – postmature.

 

Changes during fetal life

•        Relative slowdown in growth of the head compared with the rest of the body.

§  3^rd month – head: CRL is 1:1

§  5^th month – head: CHL is 1:3

§  At birth – head: CHL is 1:4

 

Changes during 3^rd month

•        Face becomes more human looking.

•        Limbs reach their relative lengths in comparison to the body size.

•        Primary ossification centers appear.

•        Intestinal loop causes a large swelling in the umbilical cord.

 

Changes during 4^th & 5^th months

•        Fetus lengthens rapidly: at the end of the first half of IUL, its CRL is half the total  length of newborn.

•        Fetus is covered with fine hair (lanugo hair). Eye brows & head hair is visible.

•        Fetal movements are felt by the mother.

 

Changes during 6^th month

•        Skin of the fetus is reddish & has a wrinkled appearance.

•        A fetus born before the first half of 7^th month has great difficulty in surviving as respiratory & CNS has not differentiated sufficiently.

 

Changes during last 2 month

•        Fetus obtains well rounded contours (subcutaneous fat deposition).

•        Skin is covered by whitish fatty substance.

•        At the end of the 9^th month, skull has the largest circumference of all parts of the body.

•        At the time of birth: ~ weight- 3000 -3400g,

            CRL- 36cm, CHL – 50 cm

 

Applied aspects

•        Low birth weight baby - > 2500g

•        Small for gestational age (SGA) – birth weight less than 2 standard deviation for their gestational age.

•        Intrauterine growth retardation (IUGR) - birth weight below the 10^th percentile for their expected birth weight for the gestational age.

 

IUGR

•        Maternal causes

§  Malnutrition

§  Maternal diseases – hypertension, heart and renal diseases

§  Toxins, alcohol, smoking

•        Fetal causes

§  Structural anomalies - CVS

§  Chromosomal anomalies –trisomy 13, 18, 21, turner syndrome

§  Infectious agents –toxoplasmosis, rubella

§  Multiple pregnancies – twins, triplets

 

Prenatal diagnosis

•        Refers to the diagnosis of genetic disorders in established pregnancies.

•        Useful in reproductive decision making:

§  termination of pregnancy when severe anomalies incompatible with post natal life are diagnosed,

§  medical treatment: administration of drugs to correct cardiac arrhythmias,

§  surgical support etc.

 

•        Certain disease can be diagnosed using prenatal screening techniques. E.g. are:

§  Neural tube defects

§  Down syndrome

Neural tube defects

•        Pregnancies with NTD are detectable at 16 weeks by assay of alfa fetoprotein in amniotic fluid or in maternal serum.

•        Maternal screening with preconceptional dietary supplementation of folic acid has resulted in 25 fold decrease in NTD.

 

Down syndrome

•        The combination of maternal age and biochemical tests (AFP, unconjugated oestriol, hCG) at 16 weeks on maternal serum can identify pregnancies with down syndrome.

•        Results confirmed by cytogenetic testing.

 

Prenatal screening techniques

ü Non invasive techniques

ü Invasive techniques

 

Non invasive procedures

ü Ultrasound scanning - Anencephaly, cleft lip & palate, microphthalmia etc are detectable.

ü X-rays – should be avoided because of mutagenic injury. Used to asses fetal skeletal dysplasia.

ü MRI – brain malformations

ü Screening of maternal blood – useful indicator for AFP (for NTD) , hCG, unconjugated oestriol for Down syndrome.

 

Invasive procedures

ü Chorionic villus sampling (10-12 weeks)

ü Amniocentesis (16-18 weeks)

ü Cordocentesis (18 weeks onwards)

ü Fetoscopy (18-20 weeks)

ü Embryo biopsy