The Collaborative International Dictionary of English v.0.48:

foetus \foetus\, Foetus \F[oe]"tus\, n.
   Same as Fetus.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fetus \Fe"tus\ (f[=e]"t[u^]s), n.; pl. Fetuses
   (f[=e]"t[u^]s*[e^]z). [L. fetus, foetus, a bringing forth,
   brood, offspring, young ones, cf. fetus fruitful, fructified,
   that is or was filled with young; akin to E. fawn a deer,
   fecundity, felicity, feminine, female, and prob. to do, or
   according to others, to be.]
   The young or embryo of a vertebrate animal in the womb, or in
   the egg; often restricted to the later stages in the
   development of viviparous and oviparous animals. showing the
   main recognizable features of the mature animal, embryo
   being applied to the earlier stages. [Written also
   f[oe]tus.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

foetus
    n 1: an unborn or unhatched vertebrate in the later stages of
         development showing the main recognizable features of the
         mature animal [syn: fetus, foetus]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FOETUS, med. jur. The unborn child. The name of embryo is sometimes given to
it; but, although the terms are occasionally used indiscriminately, the
latter is more frequently employed to designate the state of an unborn child
during the first three months after conception, and by some until
quickening. A foetus is sometimes described by the uncouth phrase of infant
in ventre sa mere.
     2. It is sometimes of great importance, particularly in criminal law,
to ascertain the age of the foetus, or how far it has progressed towards
maturity. There are certain signs which furnish evidence on this subject,
the principal of which are, the size and weight, and the formation of
certain parts as the cartilages, bones, &c. These are not always the same,
much of course must depend upon the constitution and health of the mother,
and other circumstances which have an influence on the foetus. The average
length and weight of the foetus at different periods of gestation, as
deduced by Doctor Beck, from various observers, as found by Maygrier, is
here given.

          ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
          ³     Beck.      ³  Maygrier.    ³    Beck.      ³  Maygrier.    ³
          ³                ³               ³               ³               ³
          ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
          ³             Length.            ³              Weight.          ³
          ³                                ³                               ³
ÚÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³30 days. ³3 to 5 lines.   ³10 to 12 lines.³               ³9 to 10 grains.³
³ 2 Months³2 inches.       ³4 inches.      ³2 ounces.      ³5 drachms.     ³
³ 3   do. ³3« inches.      ³6 inches.      ³2 to 3 ounces. ³2« ounces.     ³
³ 4   do. ³5 to 6 inches.  ³8 inches.      ³4 to 6 ounces. ³7 to 8 ounces. ³
³ 5   do. ³7 to 9 inches.  ³10 inches.     ³9 to 10 ounces.³16 ounces.     ³
³ 6   do. ³9 to 12 inches. ³12 inches.     ³1 to 2 pounds. ³2 pounds.      ³
³ 7   do. ³12 to 14 inches.³14 inches.     ³2 to 3 pounds. ³3 pounds.      ³
³ 8   do. ³16 inches.      ³16 inches.     ³3 to 4 pounds. ³4 pounds.      ³
ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

     3. The discordance apparent between them proves that the observations
which have been made, are only an approximation to truth.
     4. It is proper to remark that the Paris pound poids de marc, which was
the weight used by Maygrier, differs from avoirdupois weight used by Dr.
Beck. The pound poids de marc, of sixteen ounces, contains 9216 Paris
grains, whilst the avoirdupois contains only 8532.5 Paris grains. The Paris
inch is 1.065977 English inch. Vide, generally, 1 Beck's Med. Jur. 239; 2
Dunglison's Human Physiology, 391; Ryan's Med. Jur. 137; 1 Chit. Med. Jur.
403; I Briand, Med. Leg. prem. partie, c. 4, art. 2; and the articles Birth;
Dead Born; Foeticide; In ventre sa mere; infanticide; Life; and Quick with
child.