Quickening (medical)
In pregnancy terms, the moment of quickening refers to the initial motion of the fetus in the uterus as it is perceived or felt by the pregnant woman. According to the Oxford English Dictionary, to "quicken" means "to reach the stage of pregnancy at which the child shows signs of life."  In the twentieth century, ultrasound technology made it possible to see that a fetus is in motion even if the pregnant woman does not yet feel it.  This technological development made the concept of "quickening" a bit more complex.

# Medical facts
The first natural sensation of quickening may feel like a light tapping, or the fluttering of a butterfly.  These sensations eventually become stronger and more regular as the pregnancy progresses.  Sometimes, the first movements are misattributed to gas or hunger pangs.
A womans uterine muscles, rather than her abdominal muscles, are first to sense fetal motion.  Therefore, a womans body weight usually does not have a substantial effect on when movements are initially perceived.  Women who have already given birth have more relaxed uterine muscles that are consequently more sensitive to fetal motion, and for them fetal motion can sometimes be felt as early as 14 weeks.
Usually, quickening occurs naturally at about the middle of a pregnancy. A woman pregnant for the first time (i.e. a primiparous woman) typically feels fetal movements at about 20-21 weeks, whereas a woman who has already given birth at least two times (i.e. a multiparous woman) will typically feel movements around 18 weeks.

# Legal history
The word "quick" originally meant "alive".  Historically, quickening has sometimes been considered to be the beginning of the possession of "individual life" by the fetus.  British legal scholar William Blackstone explained the subject of quickening in the eighteenth century, relative to feticide and abortion:
Nevertheless, quickening was only one of several standards that were used historically to determine when the right to life attaches to a fetus.  According to the "ancient law" mentioned by Blackstone, another standard was formation of the fetus, which occurs weeks before quickening.  Henry Bracton explained the ancient law, about five hundred years before Blackstone:
The rule that a fetus was considered alive upon formation dates back at least another millennium before Bracton.  For example, in the Septuagint text of the Old Testament, killing the fetus was considered to be taking a life, "if it be perfectly formed".  Thus, quickening perceived by a woman has been only one of the standards used to mark when a human life legally begins.  Others include viability, birth, and conception.
In the 18th and 19th centuries a woman convicted of a capital crime could claim a delay in her execution if she were pregnant. In Ireland on 16 March 1831 Baron Pennefather in Limerick stated that pregnancy was not alone sufficient for a delay but there had to be quickening.  See Limerick Evening Post and Clare Sentinel 18 March 1831.

# Footnotes
-  The Compact Edition of the Oxford English Dictionary, Volume II (Oxford U. Press 1971).
-  Harms, Roger.
Mayo Clinic Guide to a Healthy Pregnancy, page 480 (HarperCollins 2004).  Retrieved 2007-02-15.
-  Van Der Ziel, Cornelia & Tourville, Jacqueline.  Big, Beautiful & Pregnant: Expert Advice And Comforting Wisdom for the Expecting Plus-size Woman (Marlowe 2006).  Retrieved 2007-02-15.
-  Levene, Malcolm et al. Essentials of Neonatal Medicine (Blackwell 2000), page 8.  Retrieved 2007-02-15.
-  Blackstone, William. Commentaries, 1:120--41 (1765).
-  Bracton, Henry. 2 On The Laws and Customs of England, 341 (S.E. Thorne trans., George E. Woodbine ed. 1968) (1250 A.D. or thereabouts).
-  "Exodus", Septuagint LXX in English, translated by Lancelot Brenton. (Bagster & Sons 1851).  The Septuagint text was written in 285-246 BC.