Who was the first to accuse enemy troops of impaling babies, cutting fetuses from women's bellies, etc.?

Long story short: Most such stories are exaggerated or pure fiction. But some are true, and such practices no doubt go back before the beginning of the species Homo sapiens.

Nobody was ever accused of impaling children and babies or anyone else on swords, spears, pikes, or bayonets until swords, spears, pikes, or bayonets were invented.

The first known bayonets were used about 1600. That martinet, General Jean Martinet (killed 1672), introduced bayonets to the French army and they were used in most European armies by the 1660s.

The first sword-like weapons date to about 3300 BC while the first true swords date to about 1700 BC in Minoan Crete.

Nobody knows when spears were first invented or which species first invented them. Early primates may have observed proboscideans using their tusks to spear enemies and copied them.

Pre human primates including Homo heidelbergensis used sharpened wooden spears at least as early as 400,000 BC. Neanderthals made stone spearheads as early as 300,000 BC and fire hardened wooden spears were used as early as 250,000 BC.

Homo sapiens began to make spears with stone spearheads about 200,000 BC.

Both chimps and orangutans have been observed spearing prey with modified tree branches.

Since spears were among the first weapons ever used by humans to hunt they were among the first weapons ever used by humans to fight other humans in inter tribal wars and among the first weapons ever used by humans to massacre defenseless humans.

Thus I expect that allegations, both true and false, that people have impaled babies on spears go back to the invention of speech complex enough to express that idea.

As for cutting pregnant [w]omen open, no doubt people began slashing other people open with stone knives if they were sharp enough, or with metal daggers and swords as soon as they [w]ere invented. Thus stories about pregnant women being cut open and their babies removed no doubt go back a long, long time.

added 04-02-2018. For example, the teenage Japanese monarch Buretsu (reigned c. 498-508) is said to have cut open a pregnant woman to examine her fetus in a book written about 200 years later.

Long ago I read an article by Mark Twain in which he mentioned in passing an alleged atrocity during the Minnesota Sioux uprising in 1862, a girl crucified by being pinned to a barn door. Since there have been wooden barn doors for centuries and millennia, no doubt stories, both true and false, have been told about people being attached to barn doors for many centuries.

One early (medieval) example is in Historia Anglorum by Henry of Huntington:

Scots: "cleft open pregnant women, and took out the unborn babes; they tossed children upon the spear-points, and beheaded priests on altars: they cut the head of crucifixes, and placed them on the trunks of the slain; and placed the heads of the dead upon the crucifixes. Thus wherever the Scots arrived, all was full of horror and full of savagery."

He wrote in the end of 11th century. Since then such accusations are often almost literally repeated.

In the earlier times, in Antiquity, when writers mentioned atrocities, they usually did not emphasize babies and pregnant women (except in the Bible). It is much more common to read that just all inhabitants, sometimes including animals, were slaughtered. It seems that people cared less about babies and women.

So it is possible that the idea comes from the Bible, and then spread together with Christianity and Islam. Can anyone find this kind of story in pagan writers?

EDIT. Here is a much earlier example:

"in their barbaric frenzy they even snatched children from their mother's breasts and dashed the guiltless infants to the ground. They held others by the feet,upside down, and cut them in two."

This is Victor of Vita, 429 a.d., describing Vandals invasion of Africa. Christian writer, of course.

There are a number of Biblical (Old Testament, so over 2000 years ago) mentions of such things, for instance Hosea 13:16, 2 Kings 15:16, Joshua 6:20-21, 1 Samuel 15:3, Psalms 137:8-9... The difference is that it's not accounts of one's enemies doing it to your people, but instructions for what your people should do to their enemies.