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Over the decades we've learned a lot about prenatal development and how exposure to toxins can negatively influence development. Many often wonder why there are so many different effects of toxins on development. For example, two women can be exposed to the same toxin and their children may show very different effects. Why? There are many reasons, but one that is especially important one is when the exposure occurred.

It is commonly believed that exposure to toxins can cause the most harm to fetuses at the end of pregnancy, when the developing baby is closest to being a newborn. That is not the case, in fact, it is the exact opposite. Here's why:

Prenatal development can be broken down into three stages: germinal, embryonic, and fetal. The germinal stage is the first stage and lasts nearly the first two weeks after conception. At this time the fertilized egg is traveling down the fallopian tubes to the uterus, where it will become implanted and develop over the course of the pregnancy. At this time, the fertilized egg is not physically connected to the mother. Toxins have relatively no effect.

The second stage, the embryonic stage, from about two to eight weeks is critical to development. This is when exposure to toxins has a particularly traumatic effect on development, sometimes ceasing it all-together. During the embryonic stage, all of the organs in the body take shape and form, so exposure to toxins during this stage can result in malformed organs.

The final stage of prenatal development is the fetal stage. This lasts from about 8 weeks until birth. At the beginning of this stage, all the organs are formed. During the fetal stage of development, they grow. Exposure to toxins during this stage usually does not result in malformations of organs, but slows growth. Exposure to toxins may result in a low birth weight infant, which can place a baby at risk for a variety of difficulties in development. The organ that grows the most during the fetal stage is the brain. Exposure to toxins during the fetal stage can affect the brain's growth and result in mental handicaps.

So the timing of exposure to toxins is crucial to understanding the effect on prenatal development. Exposure within the first week or so after conception is unlikely to have any effect. During the embryonic period, exposure to toxins may result in organ malformations. Exposure during the fetal period usually results in growth problems and negatively effects neural development.