Let's start out small.

Let's start on this post by "Grave_n_idle":

"A foetus is a foetus. A child is a child. By definition."

Well, let's do a little research on that.

http://wordnetweb.princeton.edu/perl/webwn?s=child
S: (n) child, kid (a human offspring (son or daughter) of any age) "they had three children"; "they were able to send their kids to college"

http://wordnetweb.princeton.edu/perl/webwn?s=baby
S: (n) baby (an unborn child; a human fetus) "I felt healthy and very feminine carrying the baby"; "it was great to feel my baby moving about inside"

http://dictionary.reference.com/browse/child
4. a human fetus.

http://dictionary.reference.com/browse/baby
5. a human fetus.

http://www.merriam-webster.com/dictionary/baby
1 a (1) : an extremely young child;

http://www.merriam-webster.com/dictionary/child
1 a : an unborn or recently born person
4 a : a son or daughter of human parents

http://www.thefreedictionary.com/baby
1b. An unborn child; a fetus.

http://www.thefreedictionary.com/child
2a. An unborn infant; a fetus.

http://www.yourdictionary.com/baby
1. a very young child; infant

http://www.yourdictionary.com/child
2. an unborn offspring; fetus

http://www.yourdictionary.com/fetus
an unborn offspring of a vertebrate animal that is still in the uterus or egg, esp. in its later stages and specif., in humans, from about the eighth week after conception until birth

http://www.google.com/dictionary?langpair=en|en&q=baby&hl=en&aq=f
1. A baby is a very young child, especially one that cannot yet walk or talk.
2. A baby animal is a very young animal.

http://www.google.com/dictionary?langpair=en|en&q=child&hl=en&aq=f
1. A child is a human being who is not yet an adult.
2. Someone's children are their sons and daughters of any age.

http://dictionary.infoplease.com/baby
5. a human fetus.

http://dictionary.infoplease.com/child
2. a son or daughter: All my children are married.
3. a baby or infant.
11. with child, pregnant: She's with child.

http://www.dictionary.net/baby
An infant or young child of either sex; a babe. [1913 Webster]

http://www.dictionary.net/child
CHILD, CHILDREN, domestic relations. A child is the son or daughter in relation to the father or mother.

Dictionary extension for Chrome ("child")
1. A daughter or son.
2. A prepubescent person, a minor.

http://www.enchantedlearning.com/Bisfor.shtml
A baby is a very young person.

http://www.enchantedlearning.com/Chisfor.shtml
A child is a young person.

http://www.enchantedlearning.com/Pisfor.shtml
A person is a human being.

http://dictionary.factmonster.com/child
2. a son or daughter: All my children are married.
3. a baby or infant.
11. with child, pregnant: She's with child.

http://dictionary.factmonster.com/baby
5. a human fetus.

http://www.ldoceonline.com/dictionary/child
1young person someone who is not yet an adult [= kid]:
2son/daughter a son or daughter of any age:

http://www.ldoceonline.com/dictionary/baby_1
1young child a very young child who has not yet learned to speak or walk
Collocations: -unborn baby (=a baby still inside its mother)
- the effects of alcohol on the unborn baby

http://kids.yahoo.com/reference/dictionary/english/entry/baby
An unborn child; a fetus.

http://kids.yahoo.com/reference/dictionary/english/entry/child
An unborn infant; a fetus.
A son or daughter; an offspring.

http://kids.yahoo.com/reference/dictionary/english/entry/fetus
The unborn young of a viviparous vertebrate having a basic structural resemblance to the adult animal.
In humans, the unborn young from the end of the eighth week after conception to the moment of birth, as distinguished from the earlier embryo.

http://kids.yahoo.com/reference/dictionary/english/entry/young
Offspring; brood: a lioness with her young.

http://dictionary.babylon.com/child/
kid; young person; son or daughter;
1. a son or a daughter; a male or female descendant, in the first degree; the immediate progeny of human parents

http://dictionary.babylon.com/baby/
very young child

http://lookwayup.com/lwu.exe/lwu/d?s=f&w=baby#n/7094006
3. [n] a very young mammal : "baby rabbits".

http://lookwayup.com/lwu.exe/lwu/d?s=f&w=child#n/7153837
2. [n] a human offspring (son or daughter) of any age ; "they had three children"; "they were able to send their kids to college".



I just grabbed those from the first 4 pages of a Google search for "dictionary". I could keep going, but you get the point. Even if "child" is not explicitly defined to include the human fetus, the human fetus is still covered under the "offspring of any age" definition.

Basically, Grave_n_idle's claim is as true as "A square is a square. A rectangle is a rectangle. By definition.".