God knows what any female bears [in her womb], and by how much the wombs may fall short [in gestation], and by how much they may increase [the average period]:18
for with Him everything is [created] in accordance with its scope and purpose.19
The term untha denotes any female being, whether human or animal. The "falling short" may refer either to a shortening of the usual period of gestation (e.g., to seven months in human beings) or to a falling short of the completion of pregnancy, i.e., a miscarriage; it is to be noted that the noun ghayd signifies "an abortive foetus" (Taj al-'Arus), i.e., in human beings, a foetus less than seven months old. The "increase", on the other hand, may mean either the completion of gestation or its being in excess of the average period (as, for instance, the occasional lengthening of human pregnancy from the usual period of about 280 days to 305 or, according to some medical authorities, even 307 days). In addition to this, God's knowledge of "what any female bears [in her womb]" obviously relates also to the sex of the unborn embryo as well as to the number of offspring involved in one pregnancy. - As the sequence shows, this reference to the mysteries of gestation, fully known only to God, is meant to bring out the idea that He who knows what is in the wombs knows also the innermost disposition of every human being and the direction in which that human being will develop.
Lit., "according to a measure" (bi-miqdar) - i.e., in accordance with the particular purpose for which it has been created, the exigencies of its existence and the role which it is destined to play within God's plan of creation.