The Collaborative International Dictionary of English v.0.48:

Gestation \Ges*ta"tion\, n. [L. gestatio a bearing, carrying,
   fr. gestare to bear, carry, intens. fr. gerere, gestum, to
   bear: cf. F. gestation. See Gest deed, Jest.]
   [1913 Webster]
   1. The act of wearing (clothes or ornaments). [Obs.]
      [1913 Webster]

   2. The act of carrying young in the womb from conception to
      delivery; pregnancy.
      [1913 Webster]

   3. Exercise in which one is borne or carried, as on
      horseback, or in a carriage, without the exertion of his
      own powers; passive exercise. --Dunglison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gestation
    n 1: the period during which an embryo develops (about 266 days
         in humans) [syn: gestation, gestation period]
    2: the state of being pregnant; the period from conception to
       birth when a woman carries a developing fetus in her uterus
       [syn: pregnancy, gestation, maternity]
    3: the conception and development of an idea or plan

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

GESTATION, med. jur. The time during which a female, who has conceived,
carries the embryo or foetus in her uterus. By the common consent of
mankind, the term of gestation is considered to be ten lunar months, or
forty weeks, equal to nine calendar months and a week. This period has been
adopted, because general observation, when it could be correctly made, has
proved its correctness. Cyclop. of Pract. Med. vol. 4, p. 87, art.
Succession of inheritance. But this may vary one, two, or three weeks. Co.
Litt. 123 b, Harg. & Butler's, note 190*; Ryan's Med. Jurisp. 121; Coop.
Med. Jur: 18; Civ. Code of Louis. art. 203-211; 1 Beck's Med. Jur. 478. See
Pregnancy.