Moral development takes place over a pretty long period of time. By the time most people are teens, they still often have a pretty limited understanding of moral issues: they're just starting to think of moral problems in terms of underlying abstractions. Of course, it depends on what you consider a "conscience" to be. Its possible to condition a child to react guiltily when they break a rule. But to me, that hardly qualifies as "conscience" -- you can do the same thing with a dog (but not a cat, for some reason! ;-) In my view, true conscience doesn't show up -- if it shows up at all -- until late 20's or early 30's for most people. That's the time when there's enough moral, emotional, and cognitive development to start making deep choices about personal values.

Do you perhaps mean "consciousness" --as in, awareness and self-awareness -- rather than "conscience", the ability to understand moral judgements? If you mean consciousness, then there is some debate over that, as well. Brain waves first become detectable at around 16 weeks development, so a fetus cannot possibly have consciousness before then.* However, it is unlikely that there could be a full consciousness for some weeks after that; it's generally only after week 20 or so that the fetus responds to sounds and has a startle reflex, and the brain waves which *can* be detected at this point are most like the brainwaves that are displayed during deep sleep or dream states. Even after a baby is born, the first few months are likely to be very fuzzy and unfocused. This represents a very rapid period of brain growth, with synaptic connections being formed every day. During this period of time, however, the senses (especially vision) are still maturing, and almost everything in a baby's brain is focused around immediate sensory input, and making it into something relatively stable and coherent. It is nothing at all like the consciousness displayed by adults or even older children -- however, it is a form of consciousness. At this point it just comes down to individuals arguing over definitions. --------------- Side note: Almost every pro-life website and resource on the planet carries the claim that brain activity is detectable in week 6, or at around 40 days. This is based on a single paper which was published in 1964, which claimed to have measured EEG activity in the fetus at day 40 (H. Hamlin, "Life or Death by EEG," JAMA, Oct. 12, 1964). What is not mentioned is that this paper was discredited very soon after by some medical students who, using the same equipment and setup, measured brainwaves in a bowl of Jell-o. It is, in fact, not possible for there to be brain waves at 6 weeks because there is not yet a "brain"; the embryo at week 6 is generally about an inch long and wouldn't even fill a teaspoon, and the cells which will become the brain have not yet differentiated to neurons and glia, there are no synapses or dentrites (thus no possibility of synaptic connections), and the fetus is producing no neurotransmitters at this stage. In reality, this claim has to be disregarded; there is no physical structure present which would allow brain waves to exist.