Answer to Question 1
The placental blood barrier divides the mother's blood from the fetal blood supply. The barrier prevents mixing of maternal and fetal blood cells but permits movement of tiny molecules of nutrients, gases, and waste materials. The movement of most potentially harmful substances from the mother to the fetus is prevented. Some disease-causing substances are known to be transmitted from mother to fetus, often causing birth defects or premature death. Microorganisms such as the rubella virus, HIV, and the bacteria Toxoplasma gondii, substances such as alcohol, and drugs such as Thalidomide cross the placental blood barrier and cause fetal infection or birth defects. Other toxins or microorganisms cross the placental blood barrier during birth when the placenta is torn during the birth process and maternal and fetal blood is mixed.
Answer to Question 2
Zygote, from gamete union to two days later
Embryo, from two days to eight weeks
Fetus, from eight weeks to birth
Neonate, at birth
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