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 mis-attributed to gas or hunger pangs.
A woman's uterine muscles, rather than their abdominal muscles, are first to sense fetal motion. Therefore, her 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 which are more sensitive to fetal motion; 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 primigravida woman) typically feels fetal movements at about 18–20 weeks, whereas a woman who has given birth at least once will typically feel movements around 15–17 weeks.
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:
Life... begins in contemplation of law as soon as an infant is able to stir in the mother's womb. For if a woman is quick with child, and by a potion, or otherwise, killeth it in her womb; or if any one beat her, whereby the child dieth in her body, and she is delivered of a dead child; this, though not murder, was by the ancient law homicide or manslaughter. But at present it is not looked upon in quite so atrocious a light, though it remains a very heinous misdemeanor.
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 de Bracton explained the ancient law, about five hundred years before Blackstone:
If one strikes a pregnant woman or gives her poison in order to procure an abortion, if the fetus is already formed or quickened, especially if it is quickened, he commits homicide.
In the eighteenth and nineteenth centuries, a woman convicted of a capital crime could claim a delay in her execution if she were pregnant; a woman who did so was said to "plead the belly". The law held that no women could be granted a second reprieve from the original sentence on the ground of subsequent pregnancy, even if the fetus had quickened. 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.