When is my baby alive? from conception, first heart beat or when the brain develops? and when is my baby most vunurable to teratogens?

Complex question: Though hotly debated in different circles, the medical and legal communities in the usa generally accept viability as the arbitrary point at which a baby is "alive." viability is when the baby could survive outside the mother. This is more complicated in an age with advanced medical equipment, but usually not before 24 weeks. 1st trimester is worst time for teratogens (when organs are forming).

At conception: A baby is alive after conception, but is merely a one-celled baby. At that point, the baby is a new life formed from 2 existing lives (life only comes from other life). Of course, the baby may die before it becomes a successful pregnancy. The baby is not considered a separate living being with thoughts, a conscience, certain legal rights, or an ability to live outside the womb, until months later.

Philosophical..: The first part of your question is philosophical and debated the world over. Most western countries consider the fetus to have a legal entity after the point it can potentially survive ex utero - typically 24 weeks. Of course, the conceptus is a living organism as soon as conception occurs. The maximum risk for teratogenicity is 3-8 weeks and extends to 12 weeks for some organs. Take folate (folic acid) 1 mg!