First up is ἐν γαστρί vs. ἐν τῇ κοιλίᾳ. The nouns γαστέρ ("womb") and κοιλία ("belly") are nearly synonymous at the ontological level but may differ in register. Interestingly, they exhibit different behavior when it comes to the article after ἐν in the New Testament.
γαστέρ is always bare in our corpus when used with ἐν:
κοιλία on the other hand is always arthrous:Matt 1:18 πρὶν ἢ συνελθεῖν αὐτοὺς εὑρέθη ἐν γαστρὶ ἔχουσα ἐκ πνεύματος ἁγίου.
Matt 1:23 Ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν,
Matt 24:19 οὐαὶ δὲ ταῖς ἐν γαστρὶ ἐχούσαις καὶ ταῖς θηλαζούσαις ἐν ἐκείναις ταῖς ἡμέραις
Mark 13:17 οὐαὶ δὲ ταῖς ἐν γαστρὶ ἐχούσαις καὶ ταῖς θηλαζούσαις ἐν ἐκείναις ταῖς ἡμέραις
Luke 1:31 καὶ ἰδοὺ συλλήμψῃ ἐν γαστρὶ καὶ τέξῃ υἱόν,
Luke 21:23 ⸀οὐαὶ ταῖς ἐν γαστρὶ ἐχούσαις καὶ ταῖς θηλαζούσαις ἐν ἐκείναις ταῖς ἡμέραις
1 Thess 5:3 ὥσπερ ἡ ὠδὶν τῇ ἐν γαστρὶ ἐχούσῃ, καὶ οὐ μὴ ἐκφύγωσιν.
Rev 12:2 καὶ ἐν γαστρὶ ἔχουσα· ⸂καὶ κράζει⸃ ὠδίνουσα καὶ …
If we take our point of departure from our grammars the use of the article with a proper preposition like ἐν should involve only certain, strong readings of the article, we can begin to get a sense of what kind of uses are strong. Three examples of κοιλία are possessively determined (Matt 12:29, Luke 1:41, and 44), and that appears strong enough to use the article. All of the example of γαστέρ involve being in a condition or entry into a condition, and although it could be referential, a non-referential reading relating to the condition of pregnancy seems to be in view.Matt 12:40 ὥσπερ γὰρ ἦν Ἰωνᾶς ἐν τῇ κοιλίᾳ τοῦ κήτους τρεῖς ἡμέρας καὶ
Luke 1:41 ἐσκίρτησεν τὸ βρέφος ἐν τῇ κοιλίᾳ αὐτῆς, καὶ ἐπλήσθη πνεύματος ἁγίου
Luke 1:44 ἐσκίρτησεν ⸂ἐν ἀγαλλιάσει τὸ βρέφος⸃ ἐν τῇ κοιλίᾳ μου.
Luke 2:21 πρὸ τοῦ συλλημφθῆναι αὐτὸν ἐν τῇ κοιλίᾳ.
This leaves us with Luke 2:21, where πρὸ τοῦ συλλημφθῆναι αὐτὸν ἐν τῇ κοιλίᾳ seems pretty close to Luke 1:31 καὶ ἰδοὺ συλλήμψῃ ἐν γαστρὶ in most material respects. Is this an example of the use of the article being lexically conditioned as we get in English? Maybe, but not so fast. One strong reading of the article, recognized in some of the grammars, is an anaphoric use, and Luke 2:21 appears in just such a context, one that points back to 1:31.