Stages of development, from an
ovum & spermatozoon to a newborn

Perhaps one day before conception: The woman ovulates and produces one mature ovum (a.k.a. gamate, egg cell, egg). As for the spermatozoa. it also carries a "half cargo" of human DNA -- again only 23 chromosomses, one of which is always a X sex chromosome. It travels down one of her fallopian tubes towards her uterus. It is about 1/100" in diameter, and would be barely visible to the naked eye. It also considered by most of the public to be a form of human life, for the above reasons. But it does not meet some scientists' strict definition of a living organism, because it lacks one factor: the ability by itself to reproduce. It can only reproduce with the assistance of a spermatozoon. Some of these scientists have described an ovum as an "inert globule of organic matter."

If the woman has not ovulated, has unprotected sexual intercourse, wants to avoid a pregnancy, and takes an "morning after" pill quickly, it will normally prevent ovulation. If ovulation has occurred, it will normally prevent conception.

A microphotograph of an ovum being surrounded by large numbers of spermatozoa

₆ A microphotograph of a spermatozoon fusing with an ovum

During the process of conception: One very lucky spermatozoon out of hundreds of millions ejaculated by the man may penetrate the outside layer of the ovum. This happens typically in the upper third of one of the woman's Fallopian tubes. The surface of the ovum then changes its electrical characteristics and normally prevents additional sperm from entering. A genetically unique entity is formed shortly thereafter, called a zygote. This is commonly referred to as a "fertilized ovum." However that term is not really valid because the ovum ceases to exist after the completion of conception.

Writers often refer to the "moment of conception" or "instant of conception." Actually, this is a process that extends over time.

Half of the zygote's 46 chromosomes come from the egg's 23 chromosomes and the other half from the spermatozoon's 23. The result is a unique DNA structure, different from both that of the ovum and the spermatozoon. Thus, the resulting newborn will contain a DNA that is different from its birth mother, from its birth father, and from its siblings. These differences may give the child a reproductive advantage or disadvantage over other children in society. It is this factor that Charles Darwin made the driving force of his theory of evolution.

The zygote  "...is biologically alive. It fulfills the four criteria needed to establish biological life:

1. metabolism,
2. growth,
3. reaction to stimuli, and
4. reproduction." ₁

It can reproduce itself through twinning at any time up to about 14 days after conception; this is how identical twins are caused.

The zygote will contain an X sex chromosome donated from the egg and either an X or Y sex chromosome coming from the spermatozoon. If it ends up with XX chromosomes, the xygote is female; if XY, it is male. In this way, the sex of a zygote, embryo, fetus and child is determined by the birth father's spermatozoa. Unfortunately, in the past, women were often blamed for producing new or no male children. In some cultures, particularly those where women are devalued, they are still unjustly blamed.

Conception is the point when the vast majority of pro-life groups, conservative Christians, and others define as the beginning of pregnancy. ₈ Most of these groups also define the start of a human person as occurring at conception.

The zygote first divides into two identical cells, called blastomeres. They continue to subdivide once every 12 to 20 hours as the zygote slowly passes down the fallopian tubes. It develops into a morulla and blastocyst.

The medical definition of the start of pregnancy is about 10 days after conception, when the blastocyst implants itself in the inner wall of the uterus.

Many religious groups, Christian and others, believe that God implants a soul in the zygote during the conception process or later. Various faith groups define the soul as containing various combinations of a human's mind, will, emotions, memories, etc. Some groups regard the implantation of the soul as the defining event that changes human life into a human person. Most religious progressives and secularists note that a soul with these functions cannot exist until about the 26th week of pregnancy after the fetus becomes sentient. its higher brain functions first appear, and it becomes aware of its environment; most doubt the existence of the soul, and note that it is believed to be weightless, invisible, and undetectable by any means known to science.

It was once believed by medical researchers that If the woman has taken emergancy contraception (a.k.a. EC & the "morning after" pill) quickly after unprotected intercourse, and it has not prevented ovulation, and it has not prevented conception, then the EC may prevent the blastocyst from attaching to the wall of the womb. However, further research has shown that this third mechanism seems to be impossible. Many pro-life groups and religious conservatives still assume that the pill can prevent implantation. Since these groups generally regard pregnancy as having been started at conception, they regard emergency contraception as a possible abortifacient. Many routinely refer to it as an abortifacient.

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