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Environmental Causes of Abnormalities – Weeks 3-8 of Embryogenesis

by John McLachlan, PhD
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    00:01 As I hinted, the cause of developmental abnormalities that receives the most attention is not the genetic and chromosomal ones which actually cause most of the abnormalities where we know of a cause. It’s about factors in the environment. Again, there’s an emotional component to that.

    00:19 People are very concerned that they might have done something or been exposed to something which caused an abnormality in their child. But one important thing to understand about this, something that’s not generally well understood is that there’s generally not a specific relationship between a cause and an effect, where one cause has a particularly clear outcome on the development of the baby. Perhaps, the most famous agent which caused abnormalities or teratogen, as they are known, is thalidomide, which is a drug given to a pregnant woman, and there’s a very high likelihood of causing abnormalities.

    01:03 People often think of the thalidomide defects as being related to limbs and it’s certainly true that they affect limbs. But if you remember our diagram about frequencies, limbs are one of the things that frequently go wrong. Therefore, if you increase the total number of abnormalities, then you may well increase the number of limb abnormalities in a marked manner.

    01:30 In reality, we know that thalidomide caused a whole range of defects, and the time at which the mother was exposed to thalidomide is a significant factor. There are occasional agents which have very specific effects. So some antibiotics will specifically affect tooth development, for instance. But I would say that they are probably unusual rather than common.

    01:56 Again, bear in mind that this is only a small proportion of all causes of developmental abnormalities.

    02:02 What are the kinds of environmental factors that might be involved? I’ve classified them into three groups; biological, physical, and chemical. Biological, one example might be infection with German measles or rubella. This is something which can cause really significant abnormalities in the early stages, particularly, during the embryonic period. So, embryonic death or heart defects, eye and ear defects are all things that can commonly arise from the mother being exposed to rubella. And it’s also why it’s so important to maintain population levels of vaccination at very high levels to make sure the pregnant women are not exposed to rubella in the community. Physical forces, and although is the most common is x-rays.

    02:56 Therefore, a good practice is never to radiate, never to x-ray the abdomen of a woman of childbearing age unless you are absolutely confident that she’s not pregnant. So, a precautionary principle would say, “Do not x-ray the abdomen of a female of childbearing age,” and that could be a very wide time span since women could be pregnant from the age of 12 to the age of 60 these days. Therefore, caution is required under these circumstances.

    03:28 Then finally, there are chemical causes, and I’d named thalidomide as being perhaps the classic example of that. But of course, these still represents a relatively small proportion of the total number of developmental abnormalities. But one of the problems that can arise is that parents often feel extremely bad about the fact that their child has been born with an abnormality and even though it’s inappropriate, they may well feel personally guilty, personally at fault. If, for instance, there was a genetic cause, there is absolutely no fault or blame that should attach to the parents, but nonetheless, people will often feel that, and sometimes the attempt to discharge that through finding something else to blame. So that’s not an uncommon feature of parents of children with developmental abnormalities. They’re looking for something outside themselves to blame, even though in reality, they themselves were never to blame at all. Sometimes that takes the form of seeking compensation, but in my experience, that’s not the main driver. The main driver is this emotional need to try and discharge the inappropriate guilt they feel by finding somebody else whose fault it might appear to be. So, what can we do about this, and where this prenatal diagnosis


    About the Lecture

    The lecture Environmental Causes of Abnormalities – Weeks 3-8 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.


    Author of lecture Environmental Causes of Abnormalities – Weeks 3-8 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD


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