Stages of human life during pregnancy from
to a two-week old embryo:
The following information is approximate & intended for general information.
Every pregnancy is different. Development varies from fetus to fetus.
Do not rely on this information for personal medical decisions.
The processes leading up to the birth of a newborn baby can be divided into many steps:
Knowing these processes will hopefully help people understand WHEN human life -- any form of life with human DNA -- becomes a human person with a complete set of human rights including the right to life. Lack of agreement about when this occurs fuels most of the conflict over whether women should be allowed access to abortion, and under which circumstances.
About 1 month before conception: Almost all adult males produce thousands of spermatozoa
(a.k.a. gametes or male germ cells) each second. Through a process called "meiosis" the number of chromosomes in each spermatozoon produced is half the normal number found in human DNA: 23 instead of 46. Some spermatozoon will have an X sex chromosome; others will have a Y sex chromosome. It would take about 500 of them lined up in a row to total
1 inch in length. They take a month or so to travel from a testicle, through a long tube called the
"vas deferens," to reach a small reservoir inside the man's prostate
gland. Here, semen (a mixture of spermatozoa and various fluids) is formed.
Each spermatozoon contains human DNA, but only one complete set of 23 chromosomes; normal human cells have two sets for a total of 46.
They certainly appear to be living
organisms. As seen in a microscope, they seem to be moving energetically
with the sole motivation of fusing with an ovum -- except that they don't have a mind, and thus cannot have any motivation. A spermatozoon's
movements are due to chemical reactions.
Most people consider them
to be a form of human life, because they appear alive and contain human DNA. However, some
scientists define "life" so strictly that spermatozoon are not considered alive, because they cannot, by themselves, reproduce. Reproduction requires an ovum and fertilization.
Perhaps one day before conception: The woman ovulates and produces one
mature ovum (a.k.a. gamete, sex cell, egg
cell, egg). Similar to the spermatozoa. it also carries a "half cargo" of human DNA -- only 23 chromosomes. One of them is always a X sex chromosome. The ovum 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. A few do
reproduce later with the assistance of a spermatozoon, fertilization, followed by the ovum splitting in half to produce two ova.
Some of these scientists have
described an ovum as an "inert globule of organic matter."
If the woman has not ovulated recently, has unprotected sexual intercourse, wants to avoid a pregnancy, and takes an "morning after" pill quickly, it will normally prevent ovulation. If ovulation has already occurred, it will normally prevent conception. If conception has already occurred, medical researchers have determined that the pill will have no effect. However, many religious, social, and political conservatives have chosen to ignore the findings of the researchers, and assert -- without proof -- that the morning after pill (a.k.a. emergency contraception) can prevent implantation in the inner wall of the uterus.
An enlarged human ovum and single spermatozoon
An ovum being surrounded by large numbers of spermatozoa.
A microphotograph of a spermatozoon successfully starting to fuse 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 when the the upper
third of one of the woman's Fallopian tubes. 3 The surface of the ovum then changes its electrical
characteristics and will normally prevent 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 zygote is no longer referred to as an ovum after fertilization.
Some writers often refer to the "moment of conception" or "instant of conception." Actually, conception is a process that extends over hours.
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 human DNA structure,
different from both that of the ovum's and the spermatozoon's DNA. 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 later in life in competition with other children in the family and in society. It is this factor that Charles Darwin made the driving force of his theory of evolution. His term "survival of the fittest" means the ability of a baby to survive and later produce the most offspring.
The zygote is universally regarded as being "...
biologically alive. It fulfills the four [scientific] criteria needed to establish
- reaction to stimuli, and
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 zygote is genetically female; if XY, it is genetically male. In this way, the genetic 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 few or no male children. In some cultures, particularly those where women are devalued, they are still unjustly blamed.
Sometimes, a male zygote will be formed with a XXY 9, XYY 10, or similar sex chromosome makeup.
In rare instances, a zygote will be formed with XY sex chromosomes, be a genetic male, develop into a fetus, have male sexual organs visible at birth, be registered as a male and yet have female structures in their brain located in the "central part of the bed nucleus of the stria terminalis (BSTc)." 7,8 This will cause them to identify their gender as female later in life; they will discover that they are transgender and female. Similarly, even more rarely, a zygote will have XX sex chromosomes, develop into a fetus, be identified at birth as a female, yet have male brain BSTc structures. Later in life they will identify as a female to male transgender person (FTM).
However, the vast majority of newborns will have BSTc structures in their brain that match both thei genetic gender and their sexual organs. They are referred to as cisgender individuals. More details.
Conception is the
event when the vast majority of pro-life groups, conservative Christians, and some others define as the
beginning of pregnancy. 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
about once every 12 to 20 hours as the zygote slowly passes down the fallopian
tubes. It is subsequently referred to as a morula and later a blastocyst.
definition of the start of pregnancy occurs about 10 days after conception,
at the time that the blastocyst implants itself in the inner wall of the uterus (a.k.a. the womb).
Many religious groups, Christian and others, believe that God implants a soul in the zygote during or after the conception process. 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.
Many religious progressives and secularists note that if a soul exists it cannot function until about the 26th week of pregnancy after the fetus becomes sentient. Only then do its higher brain functions first appear, and the fetus becomes aware of its environment to some degree. Most note that a soul is believed to be weightless, invisible, and undetectable by any means known to science. Many doubt the soul's existence.
About 3 days after conception: The zygote now consists of about 16 cells and is called a morula (a.k.a. pre-embryo). It has normally reached or exited the fallopian tube and entered the uterus.
5 days or so after conception: The grouping of cells are now called a
blastocyst. A cavity appears in its center. It has an inner group of cells which will become the embryo and later
the fetus, and still later the newborn. It has an outer shell of cells which will "become the membranes
that nourish and protect the inner group of cells." 3 It has
traveled down the fallopian tubes and has started to attach itself to the endometrium, the
inside wall of the uterus. The cells in the inside of the
blastocyst, called the embryoblast, start forming the embryo. The outer
cells, called the trophoblast, start to form the placenta. 4 The blastocyst is often referred to as a "pre-embryo."
9 or 10 days after conception: The blastocyst has fully attached itself to endometrium -- the inner lining of the uterus. Primitive placental blood circulation
has begun. This blastocyst has become one of the lucky ones. The vast majority of ova are never fertilized, don't make it this far in the process and are ejected from the uterus.
Medical researchers once speculated If the woman has taken emergency 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 might prevent the blastocyst from attaching to the wall of the womb. However, further research has shown that this third mechanism appears to be impossible. That is, EC acts as a true contraceptive. Many pro-life groups and religious conservatives reject the research findings and still assume that the EC 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 actual abortifacient. In spite of manufacturers' objections, and the findings of research scientists, the U.S. federal government still requires EC packaging to state that preventing implantation is still a possibility.
12 days or so after conception: The blastocyst has
started to produce unique hormones which can be detected in the woman's urine. This is is the event that all (or almost) all
pro-choice groups and almost all physicians (who are not conservative
Christians) define to be the start of pregnancy. If instructions are followed exactly, a
home-pregnancy test will reliably detect pregnancy at this point, or shortly thereafter.
13 or 14 days after conception: A "primitive streak"
appears in the embryo. It will later develop into the fetus' spinal colum.
This is the point at which spontaneous division of the blastocyst -- the process by which identical twins are developed -- is not longer
possible. The pre-embryo is now referred to as an embryo. It is a very small
cluster of undifferentiated
cells at this stage of development. It has no functioning brain; it has no internal organs; it is not conscious at this stage, or for many months afterwards.
Whether the embryo is a human person at this state of development is a hotly contested debate:
- Most in the pro-life community say that it has been a person since conception. They often refer to pre-embryos and embryos as
- Most in the pro-choice community refer to it as an embryo and believe that personhood is only attained much later in gestation -- perhaps at birth.
This lack of agreement generates most of the conflict, heat, and anger over women's abortion access.
This topic continues in the next essay with a description of pre-natal
human development from a three-week old fetus to a newborn.
The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.
Image taken from Wikipedia at: http://en.wikipedia.org/ Author unknown. Originated at http://www.pdimages.com in public domain.
Carl Sagan, "Billions and Billions", Random House, New York NY (1997), Pages 163-179.
C. George Boeree, "General Psychology: Prenatal development," personal web site at: http://www.ship.edu/
Yuping Wang and Shuang Zhao, "Vascular Biology of the Placenta," National Center for Biotechnology, at: http://www.ncbi.nlm.nih.gov/
A frame grab from "Alexander Tsiaras: Conception to birth -- visualized," Video, TED, uploaded to YouTube on 2011-NOV-14, at: http://youtu.be/
"Spermatozoons, floating to ovule," © CanStockPhoto, at: Can Stock Photo
- Frank P.M. Kruijver, "Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus," The Journal of Clinical Endocrinology & Metabolism, Vol. 85, No. 5, at: http://sindromebenjamin.tripod.com/
- Please don't email us asking what the BSTc refers to in the brain. We haven't the foggiest idea.
"47, XXY (Klinefelter syndrome)." Learn Genetics, at: http://learn.genetics.utah.edu/
"XYY Syndrome," National Organization for Rare Disorders, 2012, at: https://rarediseases.org/
Copyright © 1997 to 2015 by Ontario Consultants
on Religious Tolerance
Latest update: 2015-DEC-25
Author: B.A. Robinson