Parental Age
at Conception and the Child’s Health
There are many ways in which
birth defects can arise.
·
In case
of malformation, the fetus or structure
is genetically abnormal and thus "programmed" to develop abnormally.
An example is limb contractures resulting from diastrophic dysplasia.
·
A deformation
is when a genetically normal fetus develops in an abnormal uterine environment,
causing structural changes. An example is oligohydramnios causing limb
contractures.
·
In a
disruption, a genetically normal fetus suffers an insult resulting in disruption
of normal development. An example is early amnion rupture causing limb
deformities. (1)
Multiple structural defects or
developmental abnormalities can also occur together in one individual. A
cluster of several anomalies or defects is called a syndrome. In this case (1) :
· All abnormalities occurred sequentially as the result of one initial insult (example: oligohydramnios leading to pulmonary hypoplasia, limb contractures, and facial deformities).
· An association where particular anomalies occur together more frequently, but do not seem to be linked etiologically (example: CHARGE association, which is combination of coloboma, heart defects, atresia choanae, mental retardation, growth deficiency, and ear anomalies).
Between 2 - 3 % children are born
with a congenital birth defect. By age 18, approximately 8 % are found to have
one or more anomalies (1) .
A major cause of these anomalies
is genetic defect. There are many reasons of gene disorders and one important
and modifiable cause of the same is parental age at the time of conception.
Increased Maternal age has been often linked with a proportional increase in
risk of fetus developing a chromosomal anomaly. One such example is that of
TRISOMY 21 also called Down syndrome after J.L.H. Down who described it in
1866. Almost 95 % of Down syndrome is due to maternal nondisjunction of
chromosome 21 (75 % during meiosis I and 25 % in meiosis II). The remaining
cases result from mosaicism or translocation (1) .
There is an increased risk of
chromosome anomalies with advancing maternal age. The observed risk of having a
live born baby with Down syndrome (in the absence of a family history) is (2) :
|
Age
|
Risk
|
|
25
|
1 in 1500
|
|
30
|
1 in 800
|
|
35
|
1 in 350
|
|
36
|
1 in 300
|
|
37
|
1 in 200
|
|
38
|
1 in 170
|
|
39
|
1 in 140
|
|
40
|
1 in 100
|
|
45
|
1 in 30
|
Once a woman has had a child with
trisomy 21(Down Syndrome) resulting from nondisjunction, her risk to have
another child with any trisomy is 1 percent until her age-related risk exceeds
this; then her age-related risk predominates (1) .
A recent study has shown that not
only the mother’s age, a child’s father’s age at the time of conception is also
an important factor in the risk of passing on new gene mutations to children
and this may help explain why childhood autism rates are rising (3) .
In the study researchers
sequenced the genomes of 78 Icelandic families with children diagnosed with
autism or schizophrenia and found that father's age was crucial to the genetic
risk of such disorders. As a man ages, the number of hereditary mutations in
his sperm also increases. This age-linked
increase in mutations proportionally increases the chance that a child will
carry a harmful mutation that may lead to conditions like autism and
schizophrenia (3) .
The study found an average of two
more new gene mutations appeared in offspring for every year of increase in a
father's age - meaning the number of new mutations passed on by fathers would
double every 16.5 years from puberty onwards (3) .
Studies in Iceland have shown
that the risk of both schizophrenia and autism increase significantly with father's
age at conception, and that men are having children later. The average age of
Iceland fathers conceiving in 2011 was 33 years, up from 27.9 years in 1980 (3) .
Demographic changes of this type
- such as men tending to have children later - are not unique to Iceland, so
suggest the reported increase in autism rates around the world was at least
partially due to older fathers (3) .
References:
|
1.
|
F. Gary Cunningham
NFGKJLLCGJCHKDW. Williams Obstetrics. 21st ed.
|
|
2.
|
[Online]. [cited
2012 august. Available from: http://www.geneticseducation.nhs.uk/learning-genetics/common-genetic-conditions/maternal-age-effect.aspx.
|
|
3.
|
Kelland K. reuters.
[Online]. [cited 2012 august. Available from: http://www.reuters.com/article/2012/08/22/us-genes-risk-autism-idUSBRE87L0RB20120822.
|
Further Reading:
|
1.
|
Zofnat
Wiener-Megnazi RAMD. [Online]. Available from: http://www.nature.com/aja/journal/v14/n1/full/aja201169a.html.
|
|
2.
|
[Online]. Available
from: http://ghr.nlm.nih.gov/condition/diastrophic-dysplasia.
|
|
3.
|
[Online]. Available
from: http://ghr.nlm.nih.gov/condition/diastrophic-dysplasia.
|
|
4.
|
[Online]. Available
from: http://en.wikipedia.org/wiki/Oligohydramnios.
|
|
5.
|
[Online]. Available
from: http://www.geneticseducation.nhs.uk/learning-genetics/common-genetic-conditions/down-syndrome.aspx.
|
|
6.
|
[Online]. Available
from: http://emedicine.medscape.com/article/942350-overview.
|
|
7.
|
[Online]. Available
from: http://en.wikipedia.org/wiki/Nondisjunction.
|
|
8.
|
[Online]. Available
from: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002494/.
|
|
9.
|
[Online]. Available
from: http://psychcentral.com/disorders/schizophrenia/.
|
|
10.
|
[Online]. Available
from: http://evolution.berkeley.edu/evolibrary/article/mutations_01.
|
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