Why do toxins cause more medical problems during the first week of preganacy rather than the later weeks?

Rapid growth: At that time the baby is just a rapidly dividing ball of cells it is vulnerable. That is the time when the embryo is rapidly growing into a fetus. There is a massive amount of cellular division and every division has to be just right. Any small damage will be amplified down the line. Once the baby is more-human like the toxins have less of an effect.