Pregnancy First Trimester- Trimesters show how your unborn child in the form of a foetus in developing with each passing week. During the first trimester the foetus is just a little more than 2.54 cm by the ninth week and one of the very important stages is the embryonic stage. In this stage the embryo is vulnerable to substances like alcohol and radiation. Likewise the first trimester requires your attention and you should be taking care of yourself well. Understand the first trimester through our articles and prepare yourself for a healthy and fruitful first trimester.