Environmental toxicants and fetal development
is the impact of different toxic substances
from the environment on the development of
the fetus.
This article deals with potential adverse
effects of environmental toxicants on the
prenatal development of both the embryo or
fetus, as well as pregnancy complications.
The human embryo or fetus is relatively susceptible
to impact from adverse conditions within the
mother's environment.
Substandard fetal conditions often cause various
degrees of developmental delays, both physical
and mental, for the growing baby.
Although some variables do occur as a result
of genetic conditions pertaining to the father,
a great many are directly brought about from
environmental toxins that the mother is exposed
to.
Various toxins pose a significant hazard to
fetuses during development.
A 2011 study found that virtually all US pregnant
women carry multiple chemicals, including
some banned since the 1970s, in their bodies.
Researchers detected polychlorinated biphenyls,
organochlorine pesticides, perfluorinated
compounds, phenols, polybrominated diphenyl
ethers, phthalates, polycyclic aromatic hydrocarbons,
perchlorate PBDEs, compounds used as flame
retardants, and dichlorodiphenyltrichloroethane
(DDT), a pesticide banned in the United States
in 1972, in the bodies of 99 to 100 percent
of the pregnant women they tested.
Bisphenol A (BPA) was identified in 96 percent
of the women surveyed.
Several of the chemicals were at the same
concentrations that have been associated with
negative effects in children from other studies
and it is thought that exposure to multiple
chemicals can have a greater impact than exposure
to only one substance.
== Effects ==
Environmental toxicants can be described separately
by what effects they have, such as structural
abnormalities, altered growth, functional
deficiencies, congenital neoplasia, or even
death for the fetus.
=== Preterm birth ===
One in ten US babies is born preterm and about
5% have low birth weight.
Preterm birth, defined as birth at less than
37 weeks of gestation, is a major basis of
infant mortality throughout childhood.
Exposures to environmental toxins such as
lead, tobacco smoke, and DDT have been linked
with an increased risk for spontaneous abortion,
low birth weight, or preterm birth.
=== Structural congenital abnormality ===
Toxic substances that are capable of causing
structural congenital abnormalities can be
termed teratogens.
They are agents extrinsic to embryo or fetus
which exert deleterious effects leading to
increased risk of malformation, carcinogenesis,
mutagenesis, altered function, deficient growth
or pregnancy wastage.
Teratogens are classified in four main categories:
Drugs and chemicals.
In addition to environmental chemicals, this
category also includes recreational and pharamaceutical
drugs in pregnancy.
Vertically transmitted infections
Radiation, such as X-rays
Mechanical forces, such as oligohydramniosTeratogens
affect the fetus by various mechanism including:
Interfering with cell proliferation rate,
such as viral infection and ionization
Altered biosynthetic pathways, as seen in
chromosomal defects
Abnormal cellular or tissue interactions,
as seen in diabetes
Extrinsic factors
Threshold interaction of genes with environmental
teratogens
=== 
Neurodevelopmental disorder ===
Neuroplastic effects of pollution can give
rise to neurodevelopmental disorders.
Many cases of autism are related to particular
geographic locations, implying that something
in the environment is complementing an at-risk
genotype to cause autism in vulnerable individuals.
These findings regarding autism are controversial,
however, with many researchers believing that
increasing rates in certain areas are a consequence
of more accurate screening and diagnostic
methods, and are not due to any sort of environmental
factor.
== Toxicants and their effects ==
Substances which have been found to be particularly
harmful are lead (which is stored in the mother's
bones), cigarette smoke, alcohol, mercury
(a neurological toxicant consumed through
fish), carbon dioxide, and ionizing radiation.
=== Alcohol ===
Drinking alcohol in pregnancy can result in
a range of disorders known as fetal alcohol
spectrum disorders.
The most severe of these is fetal alcohol
syndrome.
=== Tobacco smoke ===
Fetal exposure to prenatal tobacco smoke may
experience a wide range of behavioral, neurological,
and physical difficulties.
Adverse effects include stillbirth, placental
disruption, prematurity, lower mean birth
weight, physical birth defects (cleft palate
etc.), decrements in lung function, increased
risk of infant mortality.
=== Mercury ===
Elemental mercury and methylmercury are two
forms of mercury that may pose risks of mercury
poisoning in pregnancy.
Methylmercury, a worldwide contaminant of
seafood and freshwater fish, is known to produce
adverse nervous system effects, especially
during brain development.
Eating fish is the main source of mercury
exposure in humans and some fish may contain
enough mercury to harm the developing nervous
system of an embryo or fetus, sometimes leading
to learning disabilities.
Mercury is present in many types of fish,
but it is mostly found in certain large fish.
One well-documented case of widespread mercury
ingestion and subsequent fetal development
complication took place in the 1950s in Minamata
Bay, Japan.
Used by a nearby industrial plant in the manufacture
of plastics, methyl mercury was discharged
into the waters of Minamata Bay, where it
went on to be ingested regularly by many villagers
who used the fish living in the bay as a dietary
staple.
Soon, many of the inhabitants who had been
consuming the mercury-laden meat began experiencing
negative effects from ingesting the toxin;
however, the mercury especially impacted pregnant
women and their fetuses, resulting in a high
rate of miscarriage.
Surviving infants exposed to mercury in-utero
had extremely high rates of physical and mental
handicaps, as well as physical abnormalities
from exposure in the womb during key stages
in fetal physical development.
The United States Food and Drug Administration
and the Environmental Protection Agency advise
pregnant women not to eat swordfish, shark,
king mackerel and tilefish and limit consumption
of albacore tuna to 6 ounces or less a week.High
mercury levels in newborns in Gaza are theorized
to originate from war weaponry.Mercury exposure
in pregnancy may also cause limb defects.
=== Lead ===
Adverse effects of lead exposure in pregnancy
include miscarriage, low birth weight, neurological
delays, anemia, encephalopathy, paralysis,
blindness,The developing nervous system of
the fetus is particularly vulnerable to lead
toxicity.
Neurological toxicity is observed in children
of exposed women as a result of the ability
of lead to cross the placental barrier.
A special concern for pregnant women is that
some of the bone lead accumulation is released
into the blood during pregnancy.
Several studies have provided evidence that
even low maternal exposures to lead produce
intellectual and behavioral deficits in children.
=== Dioxin ===
Dioxins and dioxin-like compounds persists
in the environment for a long time and are
widespread, so all people have some amount
of dioxins in the body.
Intrauterine exposure to dioxins and dioxin-like
compounds have been associated with subtle
developmental changes on the fetus.
Effects on the child later in life include
changes in liver function, thyroid hormone
levels, white blood cell levels, and decreased
performance in tests of learning and intelligence.
=== Air pollution ===
Air pollution can negatively affect a pregnancy
resulting in higher rates of preterm births,
growth restriction, and heart and lung problems
in the infant.Compounds such as carbon monoxide,
sulfur dioxide and nitrogen dioxide all have
the potential to cause serious damage when
inhaled by an expecting mother.
Low birth weight, preterm birth, intrauterine
growth retardation, and congenital abnormalities
have all been found to be associated with
fetal exposure to air pollution.
Although pollution can be found virtually
everywhere, there are specific sources that
have been known to release toxic substances
and should be avoided if possible by those
who wish to remain relatively free of toxins.
These substances include, but are not limited
to: steel mills, waste/water treatment plants,
sewage incinerators, automotive fabrication
plants, oil refineries, and chemical manufacturing
plants.Control of air pollution can be difficult.
For example, in Los Angeles, regulations have
been made to control pollution, by putting
rules on industrial and vehicle emissions.
Improvements have been made to meet these
regulations.
Despite these improvements, the region still
does not meet federal standards for ozone
and particulate matter.
Approximately 150,000 births occur every year
in Los Angeles.
Thus, any effects air pollution has on human
development in utero are of great concern
to those who live in this region.Particulate
matter (PM) consist of a mixture of particle
pollutants that remain in the air, and vary
be region.
These particles are very small, ranging from
PM10 to PM 2.5, which can easily enter the
lungs.
Particulate matter has been shown to be associated
with acute cardio-respiratory morbidity and
mortality.
Intrauterine growth has been shown to be affected
by particulate matter, leading to unhealthy
outcomes for fetal development such as poor
or slow fetal growth, and increasing fetal
morbidity and mortality.
A study from 2012 found that exposures to
PM 2.5 differed by race/ethnicity, age, as
well as socioeconomic status, leading to certain
populations experiencing greater negative
health outcomes due to environmental pollution,
especially relating to particulate matter.
=== Pesticides ===
Pesticides are created for the specific purpose
of
causing harm (to insects, rodents, and other
pests), pesticides have
the potential to serious damages to a developing
fetus, should they be
introduced into the fetal environment.
Studies have shown that
pesticides, particularly fungicides, have
shown up in analyses of
infant's cord blood, proving that such toxins
are indeed transferred
into the baby's body.
Overall, the two
pesticides most frequently detected in cord
blood are diethyltoluamide
( a commonly used repellant) and vinclozolin
(a fungicide).
Although pesticide toxicity is not as frequently
mentioned as some of the other methods of
environmental toxicity, such
as air pollution, contamination can occur
at any time from merely
engaging in everyday activities such as walking
down a pathway near a
contaminated area, or eating foods that have
not been washed properly.
In 2007 alone, 1.1 billion pounds of
pesticides were found present in the environment,
causing pesticide
exposure to gain notoriety as a new cause
of caution to those wishing
to preserve their health.
=== Benzenes ===
Benzene exposure in mothers has been linked
to fetal brain defects especially neural tube
defects.
In one study, BTEX (Benzene, toluene, ethylbenzene,
xylenes) exposure during the first trimester
of pregnancy has been clearly indicating negative
association with biparietal brain diameter
between 20 and 32 weeks of pregnancy.
Women with high exposure to toluene had three
to five times the miscarriage rate of those
with low exposure, and women with occupational
benzene exposure have been shown to have an
increased rate of miscarriages.
Paternal occupational exposure to toluene
and formaldehyde has also been linked to miscarriage
in their partners.
Normal development is highly controlled by
hormones, and disruption by man made chemicals
can permanently change the course of development.
Ambient ozone has been negatively associated
with sperm concentration in men, chemicals
associated with UOG operations (e.g., benzene,
toluene, formaldehyde, ethylene glycol and
ozone) have been associated with negative
impacts on semen quality, particularly reduced
sperm counts.A 2011 study found a relationship
between Neural Tube Defects and maternal exposure
to benzene, a compound associated with natural
gas extraction.
The study found that mothers living in Texas
census tracts with higher ambient benzene
levels were more likely to have offspring
with Neural Tube Defects, such as Spina Bifida,
than mothers living in areas with lower benzene
levels.
=== Other ===
Heat and noise have also been found to have
significant effects on development.
Carbon dioxide – decreased oxygen delivery
to brain, intellectual deficiencies
Ionizing radiation – miscarriage, low birth
weight, physical birth defects, childhood
cancers
Environmental exposure to perchlorate in women
with hypothyroidism causes a significant risk
of low IQ in the child.
== Avoiding relevant environmental toxins
in pregnancy ==
The American College of Nurse-Midwives recommends
the following precautions to minimize exposure
to relevant environmental toxins in pregnancy:
Avoiding paint supplies such as stained glass
material, oil paints and ceramic glazes, and
instead using watercolor or acrylic paints
and glazes.
Checking the quality of the tap water or bottled
water and changing water drinking habits if
necessary.
If living in a home built before 1978, checking
whether lead paint has been used.
If such is the case, paint that is crumbling
or peeling should not be touched, a professional
should remove the paint and the site should
be avoided while the paint is removed or sanded.
To decrease exposure to pesticides; washing
all produce thoroughly, peeling the skin from
fruits and vegetables or buying organic produce
if possible.
Avoiding any cleaning supply labeled "toxic"
or any product with a warning on the label,
and instead trying natural products, baking
soda, vinegar and/or water to clean.
== Natural gas development ==
In a rural Colorado study of natural gas development,
maternal residence within a 10-mile radius
of natural gas wells was found to have a positive
association to the prevalence of congenital
heart defects (CHDs) and neural tube defects
(NTDs).
Along with this finding, a small association
was found between mean birth weight and the
density and proximity to the natural gas wells.
Maternal exposure through natural gas wells
may come in the form of benzene, solvents,
polycyclic aromatic hydrocarbons (PAHs), and
other air pollutants such as toluene, nitrogen
dioxide, and sulfur dioxide.In Pennsylvania,
unconventional natural gas producing wells
increased from zero in 2005 to 3689 in 2013.
A 2016 study of 9384 mothers and 10946 neonates
in the Geisinger Health System in Pennsylvania
found prenatal residential exposure to unconventional
natural gas development activity was associated
with preterm birth and physician-recorded
high-risk pregnancy.
In Southwest Pennsylvania, maternal proximity
to unconventional gas drilling has been found
to be associated with decreased birth weight.
It was unclear which route of exposure: air,
soil or water could be attributed to the association.
Further research and larger studies on this
topic are needed.Endocrine disruptors are
compounds that can disrupt the normal development
and normal hormone levels in humans.
Endocrine-disrupting chemicals (EDCs) can
interact with hormone receptors, as well as
change hormone concentrations within the body,
leading to incorrect hormone responses in
the body as well as disrupt normal enzyme
functioning.
Oil and gas extraction has been known to contribute
to EDCs in the environment, largely due to
the high risk of ground and surface water
contamination that comes with these extractions.
In addition to water contamination, oil and
gas extraction also lead to higher levels
of air pollution, creating another route of
exposure for these endocrine disruptors.
This problem often goes under-reported, and
therefore, the true magnitude of the impact
is underestimated.
In 2016, a study was conducted to assess the
need for an endocrine component to health
assessments for drilling and extraction of
oil and gas in densely populated areas.
With the high potential for release of oil
and gas chemicals with extraction, specifically
chemicals that have been shown to disrupt
normal hormone production and function, the
authors highly emphasized the need for a component
centering around endocrine function and overall
health with health assessments, and how this
in turn impacts the environment.
== Role of the placenta ==
The healthy placenta is a semipermeable membrane
that does form a barrier for most pathogens
and for certain xenobiotic substances.
However, it is by design an imperfect barrier
since it must transport substances required
for growth and development.
Placental transport can be by passive diffusion
for smaller molecules that are lipid soluble
or
by active transport for substances that are
larger and/or electrically charged.
Some toxic chemicals may be actively transported.
The dose of a substance received by the fetus
is determined by the amount of the substance
transported across the placenta as well as
the rate of metabolism
and elimination of the substance.
As the fetus has an immature metabolism, it
is unable to detoxify substances very efficiently;
and as the placenta plays such an important
role in substance exchange between the mother
and the fetus, it goes without saying that
any toxic substances that the mother is exposed
to are transported to the
fetus, where they can then affect development.
Carbon-dioxide, lead, ethanol (alcohol), and
cigarette smoke in particular are all substances
that have a high likelihood of placental transferral.Identifying
potential hazards for fetal development requires
a basis of scientific information.
In 2004, Brent proposed a set of criteria
for identifying causes of congenital malformations
that also are applicable to developmental
toxicity in general.
Those criteria are:
Well-conducted epidemiology studies consistently
show a relationship between particular effects
and exposure to the substance.
Data trends support a relationship between
changing levels of exposure and the specific
effect.
Animal studies provide evidence of the correlation
between substance exposures and particular
effects.
== See also ==
Drugs in pregnancy
