This passage describes the "ordeal of bitter water"; if a man suspects his wife has committed adultery but he has no corroborating witnesses, she is asked to either confess guilt or make an oath of innocence before God & the religious leaders. If she pleads innocence, the priest performs a ritual and gives her water to drink. God is then expected to intervene to bring harm to the woman if she is guilty; otherwise it is understood that God has exonerated her.
The meaning of this contested passage revolves around the translation of וְנָפְלָ֖ה יְרֵכָ֑הּ (literally "and her thigh will rot" or, from the YLT, "and her thigh hath fallen"). There are 3 principal views on the meaning of this phrase:
- It literally refers to her thigh rotting--the woman will be physically injured (or die)
- It refers to a woman being/becoming barren (the curse to the guilty in verse 27 being the opposite of the blessing--conceiving--promised to the innocent in verse 28)
- It is a euphemism for a miscarriage
#1 is far and away the most common in English translations, #2 is less so, and #3 is almost unique (only found in the CEB and the NIV--and even the NIV only changed to #3 in the 2011 version--source).
If this phrase refers to a miscarriage or abortion, it is obscure; it does not use common, clearer Hebrew words to refer to a miscarriage. I am unaware of anything written before the 19th century that ever understood וְנָפְלָ֖ה יְרֵכָ֑הּ to be relevant to a discussion on abortion, and neither this phrase, nor anything like it, is used anywhere else in Biblical texts to refer to miscarriage or abortion.
Also working against interpretation 3 is the fact that nowhere in this passage on suspected adultery is it ever stated that the woman being trialed is pregnant. If the woman is found innocent, she is blessed by becoming pregnant, ergo, she wasn't already pregnant. Furthermore, there is no separate fate specified for an adulteress who is pregnant versus one who is not pregnant. If the phrase in question were explicitly a description of a miscarriage, there is a major plot hole--are only adulteresses who get pregnant punished?? The rest of the Torah would beg to differ (quite strongly, in fact).
Most translators recognize that if there is a Hebrew idiom at play here, it is uncorroborated & ambiguous; therefore, they try to translate the phrase fairly literally without adding interpretative spin that, while possible, is neither clear nor required by the text.
Why this passage says nothing about the modern abortion debate
Interpretations 1 & 2 above have nothing to do with abortion. Let's consider what does or does not follow if we were to go out on a limb and accept interpretation 3 for sake of argument.
The ancient understanding of the text
Ancient Jewish writings, such as Josephus, the Targums, and Maimonides (all far closer in language & culture to the book of Numbers than we are today) understood that the punishment being specified here is that the adulterous woman would not miscarry, but die (source). Indeed, adultery is (in most cases) considered a capital crime in the Torah.
Even earlier, the Septuagint translators (circa 200 BC) also did not leave any reference to pregnancy or miscarriage in their translation of the passage--their words can be rendered in English:
And this water that brings the curse shall enter your belly, to swell the belly and make your thigh fall to pieces (ibid, quoting A New Translation of the Septuagint, Oxford University Press, 2007)
Even if we assume that all the extant ancient writings on the passage misunderstood it, and we, sophisticated moderns are the only ones who read it correctly (I do not actually recommend making this assumption), using this passage to argue for the morality of mortals performing abortions is logically invalid.
The water given to the woman on trial is not an abortifacient; without Divine intervention the concoction would be innocuous. If she's innocent, no harm comes to her--which is, of course, unsurprising, as she hasn't ingested anything harmful. If she's guilty, God is expected to perform a miracle to demonstrate her guilt; if she's innocent, God is expected to bless her innocence. So even if this passage referred to a miscarriage (and it's not clear that it does), God is the one taking life in verse 27, just as God is the one granting life in verse 28.
That God is permitted to give & take life does not by itself say anything about whether flawed mortals may do so.
Reductio ad absurdum
David & Bathsheba's first child, conceived in adultery, was born alive and died a few days later (see 2 Samuel 12:14). The very same reasoning which starts by reading miscarriage into Numbers 5:27, and arguing from it that therefore God condones abortion, would also have to conclude that God condones killing infants after they are born.
The idea that if God did it in the Bible for reasons I do not fully understand, then I am permitted to do it as well is a really, really poor premise. Would we apply that to sacrificing one's son? The burning of Sodom & Gomorrah? The flood?
Unless we are prepared to conclude (I certainly am not) that the Bible gives mortals blanket approval to kill infants, perform human sacrifice, burn cities, and drown people who reject them, by reductio ad absurdum we cannot use Numbers 5:27 to argue that the Bible permits mortals to perform abortions either.
Numbers 5:27 has nothing to say at all, one way or the other, on the morality of abortion, nor does it promote abortion for unfaithful wives.