Human reproduction is the process by which children are conceived and born. In order for human reproduction to occur under normal circumstances, male and female must engage in sexual intercourse, which should take place within marriage. Exceptionally, reproduction may take place without intercourse via procedures such as in vitro fertilization.
Human reproduction serves to create new human beings and with each of these new human beings comes a soul. The soul appears with the person at the moment of conception, making him a complete human being from this moment. This is why abortion is murder: the abortionist has rent the soul from a living human body, the soul being the source of a full life for human beings. If hypothetically there were no soul present in the human at the time of conception and it entered the body at some later stage, then until that moment the fetus would be nothing more than an animated body, similar to how animals and plants are alive but without souls. Thus, if this were the case, abortion would not be murder, but rather a termination of a non-human entity, at least until the soul finally joined the body. However, there has never been any evidence presented that the soul can enter the body at any time later than conception. Since the soul is intrinsic to human beings, it absolutely must be present at conception and therefore abortion is murder.
There are many religious beliefs, even within Christianity, on the origin of the soul that accompanies a new human being. Some believe portions of the souls of the parents combine to form the new soul. Some believe a free-floating soul becomes trapped within the body. Some believe a soul is created with the fusion of the gametes, in a meta-chemical process. Believers in reincarnation say that the soul of a previously deceased person or animal (in this religion non-human animals are believed to have souls as well) enters the new body, so that there is a spiritual memory of the previous life.
The ultimate act of intimacy serves to unite the male and female gametes, the sperm and egg, respectively. These gametes carry just one half of the genetic material (chromosomes) from each parent, and when they combine the new embryo contains a full set of chromosomes. The process of the gametes combining is called fertilization, and takes place inside the woman's body, in the Fallopian tubes or uterus. The embryo then attaches itself to the uterine lining, and is nourished by the placenta (which in turn receives its nourishment from the mother's body) while it develops. This development takes roughly nine months (see Naegele's Rule).
At the end of this process of protected development, what was once an embryo is now born as a new baby. Human babies are relatively helpless, and depend on the care and love of both parents in order to survive to maturity themselves.
Humans are not able to reproduce alone by budding, parthenogenesis, or self-cloning as some species are capable of. In addition, human sex is fixed at conception, unlike some species'; females are not able to become male or vice versa to deal with homogeneous populations of one gender.