Timeline of Development: 

Early stages of fetal development: 

18 days:
Heart begins to beat (Even today the most common way to determine that someone is dead is probably to check for pulse or heartbeat. So if we say you're dead when your heart stops, shouldn't we say you're alive when your heart starts?)

40 days:
Brain waves can be measured with an Electronencephalogram

(Today we declare someone to be legally dead when his brain waves cease. So if the end of brainwaves marks the legal end of life, perhaps the start of brainwaves should mark the legal start of life?)

2 months:
All body systems functioning

2 months:
Ultrasound has seen babies this age sucking their thumbs

3 months:
Capable of hearing and the child breathes amniotic fluid.

4-5 months:
Baby can kick hard enough for his mother to feel it.