As I have already stated you are reading into scripture to say that what is described
Numbers 5 in any way even implies that abortion is ok in the instance of adultery. What is spoken of there is a punishment of sin. I agree that it did result in barrenness but no where does it ever even hint at the death of a child.
As to the death of an unborn child and how it was dealt with in the bible shows a high value of human life perhaps even higher than ours today rather than a lesser veiw.
Numbers 35:11-24
11 Then ye shall appoint you cities to be cities of refuge for you; that the slayer may flee thither, which killeth any person at unawares.
12 And they shall be unto you cities for refuge from the avenger; that the manslayer die not, until he stand before the congregation in judgment.
13 And of these cities which ye shall give six cities shall ye have for refuge.
14 Ye shall give three cities on this side Jordan, and three cities shall ye give in the land of Canaan, which shall be cities of refuge.
15 These six cities shall be a refuge, both for the children of Israel, and for the stranger, and for the sojourner among them: that every one that killeth any person unawares may flee thither.
16 And if he smite him with an instrument of iron, so that he die, he is a murderer: the murderer shall surely be put to death.
17 And if he smite him with throwing a stone, wherewith he may die, and he die, he is a murderer: the murderer shall surely be put to death.
18 Or if he smite him with an hand weapon of wood, wherewith he may die, and he die, he is a murderer: the murderer shall surely be put to death.
19 The revenger of blood himself shall slay the murderer: when he meeteth him, he shall slay him.
20 But if he thrust him of hatred, or hurl at him by laying of wait, that he die;
21 Or in enmity smite him with his hand, that he die: he that smote him shall surely be put to death; for he is a murderer: the revenger of blood shall slay the murderer, when he meeteth him.
22 But if he thrust him suddenly without enmity, or have cast upon him any thing without laying of wait,
23 Or with any stone, wherewith a man may die, seeing him not, and cast it upon him, that he die, and was not his enemy, neither sought his harm:
24 Then the congregation shall judge between the slayer and the revenger of blood according to these judgments:
Notice that if a man were to kill a person by accident they were not required to pay anything to anyone rather they were instructed to flee to a city of refuge and there they were safe from any recourse. However notice the way that a person is dealt with when they accidently kill an unborn child while striving with someone else.
Exodus 21:22-25
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.
23 And if any mischief follow, then thou shalt give life for life,
24 Eye for eye, tooth for tooth, hand for hand, foot for foot,
25 Burning for burning, wound for wound, stripe for stripe
Notice that even though the killing was by mistake God does not allow this person to flee to a city of refuge rather God demands punishment. Therefore it would seem that God holds the accidental killing of an unborn child as a greater crime than the accidental killing of an adult.
If God holds the accidental killing of a baby so seriously how seriously does He view the purposeful murder of an unborn baby?