placental functions and factors affecting fetal growth

Placental Functions and Factors Affecting Fetal Growth

Maternal Placental Blood Flow

Intervillous space of mature placenta contains about 150 ml of blood which is replenished 3 or 4 times a minute

Uteroplacental blood flow increases from– 50 ml per minute at 10 weeks– 500/600 ml per minute at full term

Placenta

MetabolismTransferEndocrine

Placental Transfer (gases)

Oxygen, Carbon Dioxide, Carbon Monoxide cross the placenta by simple diffusion

Placental Transfer (nutrients)

Water freely movesNo transfer of maternal cholesterol,

triglycerides or phospholipidsSmall amounts of free fatty acids transportedvitamins are essentialGlucose quickly transferred

Placental Transfer (hormones)

Protein hormones do not reach the fetus, except for the slow transfer of thryroxine and triiodothyronine

Testosterone can cross

Placental Transfer (antibodies)

Some passive immunity is conferred on the feus by the transfer of maternal antibodies (mainly gamma globulins)

diptheria, smallpox and measlesnot whooping cough and chicken pox

Glucose

Glucose is the primary source of energy for the fetal metabolism

Amino acids also requiredBoth come from the mother via the

placenta

Placental Metabolism

Particularly early in pregnancy, synthesis of glycogen, cholesterol and fatty acids

Dizygotic Twins

Monozygotic Twins

Conjoined Twins

Critical Periods

Since organogenesis occurs primarily in the embryonic period (weeks 4-8) slight influences can have drastic and irreversible effects

Sensitive periods?

Congenital MalformationsMalformations present at birth, irrespective

of cause (genetic or environmental)

Teratogens

External agents that cause congenital malformations

The Six Principles of TeratologyWilson 1959

1. Susceptibility to teratogenesis depends on the genotype of the conceptus and the manner in which this interacts with adverse environmental factors.

2. Susceptibility to teratogenesis varies with the developmental stage at the time of exposure to an adverse influence. There are critical periods of susceptibility to agents and organ systems affected by these agents.


3. Teratogenic agents act in specific ways on developing cells and tissues to initiate sequences of abnormal developmental events.

4. The access of adverse influences to developing tissues depends on the nature of the influence.

– nature of the agent– route and degree of maternal exposure– rate of placental transfer and systemic absorption– composition of the maternal and embryonic/fetal

genotypes.


5. There are four manifestations of deviant development5. Death

6. Malformation

7. Growth Retardation

8. Functional Defect)

6. Manifestations of deviant development increase in frequency and degree as dosage increases from the No Observable Adverse Effect Level (NOAEL) to a dose producing 100% Lethality (LD100).

Teratogens

Drugs and medicationsEnvironmental chemicalsIonizing radiationInfectionsMetabolic imbalance

Thalidomide

Fetal Alcohol SyndromeFetal Alcohol

Spectrum Disorder

Rubella Syndrome

Symptoms in the infant may include:

Cloudy corneas or white appearance to pupil, Deafness,

Developmental delay, Excessive sleepiness, Irritability,

Low birth weight, Mental retardation, Seizures, Small

head size, Skin rash at birth, Cardiac Anomalies

Fetal Monitoring

Ultrasonography

Monitoring: Chorionic sac during

embryonic period placental and fetal size multiple births abnormal presentations biparietal diameter

Uses reflection of very high frequency sound waves of between 3.5 to 7.0 megahertz (i.e. 3.5 to 7 million cycles per second)

Fetal Blood Sampling

Usually from the scalp, fetal blood pH is a good indicator of placental gas exchange.In the past, fetal blood sampling was used only during labor through the mother's open cervix to test blood from the fetal scalp for oxygenation. Today, in many perinatal care centers, fetal blood sampling is performed by specially trained perinatologists as part of diagnosing, treating, and monitoring fetal problems at various times during pregnancy.

Fetal Blood Sampling

A fetal blood sample may be taken to:

diagnose genetic or chromosome abnormalities.

check for and treat severe fetal anemia or other blood problems such as Rh disease.

check for fetal oxygen levels. check for fetal infection. give certain medications to the fetus.

How is fetal blood sampling performed?

A long, thin needle is inserted into the mother's uterus guided by ultrasound.

Blood may be taken from several sources:

blood vessels of the umbilical cord (also called cordocentesis, funicentesis, or percutaneous umbilical blood sampling, or PUBS)

a fetal blood vessel, usually in the liver or heart

Fetal blood transfusions may also be performed in this

used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is extracted from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for genetic abnormalities.

Little amniotic fluid present prior to 12th week of gestation

Amniocentesisalso referred to as amniotic fluid test or AFT

Chorionic Villus Sampling

chromosomal abnormalities etc.The advantage of CVS is that it can

be carried out 10-13 weeks after the last period, earlier than amniocentesis (which is carried out at 16-20 weeks).

Alpha-Fetoprotein Assay

AFP is a glycoprotein synthesized in the fetal liver and yolk sac.

The fetus normally excretes AFP into its urine, hence into the amniotic fluid.

High levels may also be present due to:– open neural tube defect – open abdominal wall defect – skin disease or other failure of the interior or exterior

body surface.– Various forms of tumours

Factors Affecting Fetal Growth

Placental Insufficiency

Placental defects effectively reduce available surface area

reduced uteroplacental blood flow may also occur due to maternal hypotension or renal disease.

Multiple Pregnancy

Individuals of multiple births usually weigh considerably less

in the third trimester placenta may not be able to supply the total requirements for multiple births

Small Babies

Low birth weight: – < 2,500g

Premature: – < 37 weeks of gestation

Small for Date: – Smaller than expected for age

placental functions and factors affecting fetal growth

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