Stages of human development, from an
ovum & spermatozoon to a newborn
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 process leading up to the birth of a newborn baby can be divided into many steps:
||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 -- 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 chromosomes; normal human cells have two.
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. Most people consider them
to be a form of human life, because they appear alive and contain human DNA. Some
scientists define "life" so strictly that spermatozoon are not considered alive, because they cannot, by themselves, reproduce. Reproduction requires an ovum and fertilization. A spermatozoon's
movements are due to chemical reactions.
Perhaps one day before conception: The woman ovulates and produces one
mature ovum (a.k.a. gamete, egg
cell, egg). As for the spermatozoa. it also carries a "half cargo" of human DNA -- again only 23 chromosomes, 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. If conception has occurred, the pill will have no effect.
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 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 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 later in life in competition with 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
- reaction to stimuli, and
- 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 zygote 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 few or no male children. In some cultures, particularly those where women are devalued, they are still unjustly blamed.
Conception is the
event when the vast majority of pro-life groups, conservative Christians, and some 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 is subsequently referred to as a morula and later a blastocyst.
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 because its higher brain functions first appear. It becomes aware of its environment to some degree. 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.
||About 3 days after conception: The zygote now consists of about 16 cells and is called a 16 cell morula (a.k.a. pre-embryo). It has normally reached or passed the
junction of the fallopian tube and the uterus.
||5 days or so after conception: A cavity appears in the center of
the morula. The grouping of cells are now called a
blastocyst. It has an inner group of cells which will become the fetus and 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 (a.k.a. womb). 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. It continues to be referred to as a
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 and don't make it this far in the process.
It was once believed by medical researchers that 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 may 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 is a true contraceptive. Many pro-life groups and religious conservatives 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 abortifacient.
||12 days or so after conception: The blastocyst has
started to produce 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 may reliably detect pregnancy at this point, or shortly thereafter.
||13 or 14 days after conception: A "primitive streak"
appears. It will later develop into the fetus' central nervous system.
This is the point at which spontaneous division of the blastocyst -- the process that sometimes produces identical twins -- 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.
||3 weeks: The embryo is now about 1/12" long, the size of a
pencil point. It most closely resembles a worm - long and thin and with
a segmented end. Its heart begins to beat about 18 to 21 days after
conception. Before this time, the woman might have noticed that
her menstrual period is late; she might suspect that she is pregnant and conduct a pregnancy test. About half of all pregnancies are unplanned. About half of unplanned pregnancies in the U.S. are terminated by an abortion.
||4 weeks: The embryo is now about 1/5" long. It looks something like a
tadpole. The structure that will develop into a head is visible, as is a noticeable tail.
The embryo has structures like the gills of a fish in the area that will later develop
into a throat.
||5 weeks: Tiny arm and leg buds have formed. Hands with webs between the fingers
have formed at the end of the arm buds. Fingerprints are detectable. The face "has
a distinctly reptilian aspect." 1 "...the
embryo still has a tail and cannot be distinguished from pig, rabbit,
elephant, or chick embryo." 3
||6 weeks: The embryo is about 1/2" long. The face has two eyes on
each side of
its head; the front of the face has "connected slits where the mouth and nose
eventually will be." 1
||7 weeks: The embryo has almost lost its tail. "The face is mammalian but
somewhat pig-like." 1 Pain sensors appear.
Many religious and social conservatives believe that the embryo at this stage can feel pain.
However, the higher functions of the brain have yet to develop, and the
pathways to transfer pain signals from the pain sensors to the brain
do not exist at this time.
||2 months: The embryo's face resembles that of a primate but is not fully human in
appearance. Some of the brain begins to form; this is the primitive "reptilian brain" that
will function throughout life. The embryo will respond to prodding, although it has no
consciousness at this stage of development. The brain's higher functions do
not develop until much later in pregnancy when the fetus becomes sentient.
||10 weeks: The embryo is now called a fetus. Its face looks human. Its gender may
be predicted from the presence or absence of a penis during an ultrasound test.
|| 13 weeks or 3 months: The fetus is about 3 inches long and weighs about an ounce.
bones can be seen. Over 90% of all abortions are performed before this stage, before the fetus has become sentient and therefore before it is conscious. 2,9
||17 weeks or 3.9 months: It is 8" long and weighs about a half pound. The fetus' movements may
begin to be
felt. Its heartbeat can usually be detected.
20 weeks or 4.6 months: Many social and religious conservatives with pro-life views assert that a fetus can feel pain at 20 weeks. They have successfully passed legislation in some states to prohibit abortions after this time. Other activists call for anesthetic during abortions of fetuses 20 weeks or later.
A bill has been filed in Congress to prohibit abortions after this point in gestation unless the woman is the victim of rape or incest or if a continued pregnancy would endanger her life. It does not contain an exception for cases in which a severe fetal abnormality is detected after 20 weeks. It is justified by the belief that a fetus can feel pain at this time -- a belief which very few medical researchers agree.
||22 weeks or 5 months: 12" long and weighing about a pound, the
fetus has hair on its head. Its movements can generally be felt. An elective abortion is usually
unavailable at this gestational age because of state and province medical
society regulations, except under very unusual circumstances. Half-way through
the 22nd week, the fetus' lungs may be developed to the point where it
would have a
miniscule chance to live on its own. State laws and medical association regulations
generally outlaw almost all abortions beyond 20 or 21 weeks gestation. "A
baby born during the 22nd week has a 14.8 percent chance of survival.
And about half of these survivors are brain-damaged, either by lack of oxygen
(from poor initial respiration) or too much oxygen (from the ventilator).
Neonatologists predict that no baby will ever be viable before the 22nd week, because before then the lungs are not fully formed." 4 Of course, if someone develops an artificial womb, then this limit could change
Fetal survival rate:"Most babies at 22 weeks are not
resuscitated because survival without major disability is so rare. A baby's
chances for survival increases 3 to 4% per day between 23
and 24 weeks of gestation and about 2 to 3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival
increases at a much slower rate because survival is high already." 5
||26 weeks or 6 months: The fetus 14" long and almost two pounds. The lungs' bronchioles develop. Interlinking of the brain's neurons begins. The higher
functions of the fetal brain turn on for the first time. Some rudimentary brain waves indicating consciousness can be detected. The fetus will probably be able to feel pain for the first time. It has become conscious to some degree of its surroundings. The fetus has become a sentient human life for the first time. Some pro-choicers define this point as the beginning of human personhood.
||7 months or 30.5 weeks: 16" long and weighing about three pounds. Regular brain waves are detectable which are similar to those in adults.
||8 months or 35 weeks: 18" long and weighing about 5 pounds.
||9 months or 39 weeks: 20" long and with an average weight of about 7 pounds,
a full-term fetus' is typically born about this time.
||Francis Beckwith, "Is the unborn human less than human?,"
ChristianAnswers.net, at: http://www.christiananswers.net/
||"Life begins at conception," Cross Publications, at: http://jesuschristsavior.net/
||B.F. Miller, "The Complete Medical Guide", Simon & Schuster, New York, NY, (1967)
||Bbwpregnancy.com has a list of Internet resources on pregnancy development at: http://www.bbwpregnancy.com/
||The Visible Embryo is a remarkable web site. It shows the various stages of development from a fertilized egg to fully-formed fetus.
||"The inside story" at StandUpGirl also shows fetal images, taken by 3-D ultrasound, 4-D ultrasound, and contact embryoscopy.
See: http://www.standupgirl.com/ Caution: the web site shows some drawings and pictures of a fetus early in pregnancy, and add an unrelated comment that describes a viable fetus near the end of pregnancy.
||Religious and social conservatives generally believe that pregnancy begins
at conception, whether achieved through sexual
intercourse or in-vitro fertilization (IVF). In the latter case, conception is
performed in a dish external to the woman's body. Typically, two dozen of the
woman's ova are harvested, and fertilized in the lab. The three or four most
vigorous pre-embryos are then selected and implanted in her uterus. The rest are
discarded, exposed to die, or frozen for possible future use.
- Carl Sagan, "Billions and Billions", Random House, New York NY (1997), Pages 163-179.
- This ultrasound picture of a fetus at 2.8 month/12 week gestation was donated by a visitor to this web site.
- C. George Boeree, "General Psychology: Prenatal development," at: http://www.ship.edu/
- Franklin Foer, "Fetal Viability," The Gist, 1997-MAY-25, at: http://www.slate.com/
- "Chances for Survival," University of Wisconsin Medical School, 2004-APR-22, at: http://www.pediatrics.wisc.edu/
- Lower image taken from Wikipedia at: http://en.wikipedia.org/
Copyright 1997 to 2015 by Ontario Consultants
on Religious Tolerance
Latest update: 2015-FEB-05
Author: B.A. Robinson