Fetus
A fetus or foetus (/ˈfiːtəs/; pl.: fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from a mammal embryo.[1] Following embryonic development, the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilization (or eleventh week gestational age) and continues until the birth of a newborn. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.
This article is about the stage of prenatal development. For other uses, see Fetus (disambiguation).Etymology[edit]
The word fetus (plural fetuses or feti) is related to the Latin fētus ("offspring", "bringing forth", "hatching of young")[2][3][4] and the Greek "φυτώ" to plant. The word "fetus" was used by Ovid in Metamorphoses, book 1, line 104.[5]
The predominant British, Irish, and Commonwealth spelling is foetus, which has been in use since at least 1594. The spelling with -oe- arose in Late Latin, in which the distinction between the vowel sounds -oe- and -e- had been lost. This spelling is the most common in most Commonwealth nations, except in the medical literature, where fetus is used. The more classical spelling fetus is used in Canada and the United States. In addition, fetus is now the standard English spelling throughout the world in medical journals.[6] The spelling faetus was also used historically.[7]
Fetus at 4+1⁄4 months
Fetus at 5 months
Immune system[edit]
The placenta functions as a maternal-fetal barrier against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur.
Maternal IgG antibodies cross the placenta, giving the fetus passive immunity against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.[27]