Oxford Languages Definition for "Embryo":
an unborn or unhatched offspring in the process of development, in particular a human offspring during the period from approximately the second to the eighth week after fertilization (after which it is usually termed a fetus).
Some choose to render the Hebrew word golem in Psalm 139:16 as "unformed substance" or something to that effect, but Brown-Driver and Briggs (BDB) Hebrew-English Lexicon tells
us that golem is a word at times used for the "embryo."
Gesenius' Hebrew-Chaldee Lexicon likewise says that golem refers to
something "rolled together" or "rude and unformed matter, not yet
wrought, the parts of which are not yet unfolded and developed." It then
states that the word is used "of the embryo." The word occurs one
time in the Hebrew Bible.
Nancy Declaisse-Walford prefers the translation "unshaped form" for the Hebrew word, which she points out is a hapax legomenon (Latin for a saying that happens once). She writes: "In Babylonian Aramaic, the word is used to designate a formless mass or an incomplete vessel. The Syriac word galmā means 'uncultivated soil.' "
Declaisse-Walford is critical of the translation, "embryo" because she thinks it is too precise and potentially misleading, probably in light of what the ancients knew about embryology. However, as we have seen, two
lexicons (BDB and Gesenius) give embryo as the term's potential sense, even if
that is not the strict meaning. Some translations opt for "before I was
born" language in Psalm 139:16 (NET Bible). Yet see the entry for golem in HALOT.
Having said the foregoing, I see nothing wrong with the NWT handling of the verse: it communicates what we would understand by "unformed substance" in the womb. As you all know, when it comes to babies, there are even finer distinctions we could make, like talk about the blastocyst. But none of these tangential matters were likely David's inspired concern.