So I went and read Numbers 5. Thank You! I haven't read that in several months!
I take it you are assuming the bitter water is an abortive elixir of some sort? It never says that. Nothing about fruit of the womb departing, anything being expelled, or dying, or even pregnancy, such as "found with child."
It appears to be a curse that would fall from the LORD if the woman had cheated on her husband, via a miraculous means, as the mixture is holy water and dust from the floor of the Tabernacle. Doesn't exactly sound like a poisonous mixture, but let's see.
What herbs and incense were used that might be on the floor in small amounts, IF any may have spilled there:
Myrrh was used in the anointing oil, and myrrh can cause contractions.
Frankincense is no danger
Stacte is a resin, nothing about it being dangerous.
Onycha: we're not really sure what that was, from a flower, a snail, or is spikenard, and spikenard may cause miscarriage
Galbanum is an emmenagogue, which encourages menstruation, hence it is contraindicated for pregnancy.
Now, given that IF there were spilled incense on the floor mixed in with the dust, and most certainly there'd have been the oil as Psalm 133:2 indicates it ran down Aaron's head to his beard, so it was dripping, then there is a possibility the dust could have caused a miscarriage.
However, there's other things at play here which I would think show the mixture was not an abortive. The thigh to rot and the belly to swell was the curse, which sounds more like the woman would be accursed and defiled, so no one could touch her, if she had committed adultery.
There's also the fact that a woman doesn't get pregnant every time she has sexual relations, so in many cases a woman could sleep with another man and if she wasn't pregnant from him, this mixture obviously wouldn't cause an abortion. If she was pregnant, it may well have done so considering the symptoms, but that would only be a secondary affect of the curse. In that regard it would be a child conceived by adultery, which in David & Bathsheba's case their child died. God took the child as direct chastisement for their sin. (2 Samuel 12:14-19) God is sovereign and does as He pleases, and whatsoever He does is good.
This was also a ceremony under the Law which was commanded by God directly to Moses.
There is also Exodus 21:22, "If men strive, and hurt a woman with child, so that her fruit depart from her, and yet no mischief follow: he shall be surely punished, according as the woman's husband will lay upon him; and he shall pay as the judges determine."
Here someone that causes a miscarriage is punished, albeit according to what the woman's husband desires the punishment to be.
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