During human embryogenisis what factors are responcible for birth defects

http://www.learn.ppdictionary.com/prenatal%20development.htm

You might need to look up the word teratogen, because that's essentially what you're referring to.


An ultrasound can reveal the size of the baby after 6 weeks (and therefore estimate their gestational age). Later ultrasounds (after 18 weeks) can show the position that the baby is lying in and detect many obvious physical abnormalities (such as a cleft lip or a heart defect). Ultrasounds cannot reveal a baby's genetic 'make-up' and this type of information can only be obtained through genetic tests . However, up to 40 to 50% of babies with genetic disorders will be detected through 'soft signs' picked up by the ultrasonographer after 18 weeks (such as a thickened fold of skin at the back of the baby's neck, which may or may not indicate a genetic disorder). Ultrasounds are also able to show the location of the placenta and possibly shed light on other issues such as the cause of bleeding during pregnancy or detecting multiple pregnancies (twins or more).

Ultrasound images are generally limited by the scope of the probe and where the sound waves can reach. Therefore during the first half of pregnancy, the whole baby can be seen on the screen, often moving their arms and legs, but the details of their face are usually less distinct. During the last half of pregnancy, the whole baby cannot be viewed all at once, but you can usually see clearer images of their facial features, hands or feet.