In Ovid's Heroides he slips in near the end that Dido was pregnant with Aeneas' child, thereby implicating that Aeneas not only killed her, but their child, when he forced her hand into suicide. Did he fashion this himself in order to heighten his dramatic portrayal of her?
It certainly looks like it. Bear in mind that the story of Dido's liaison with Aeneas is not exactly a traditional one: Naevius' Punica seems to have made them contemporaries, but other than that Ovid's only model is Vergil. So if it's not there in Vergil, it's Ovid that added it.
You can see the impact of Vergil in many parts of the poem, in places where Ovid echoes and expands on Vergil: e.g. Her. 7.37-8 imitates Vergil's Homeric echo in Aeneid 4.365-7, but makes the echo more exact; Her. 7.54 alludes to the storm in Aeneid 1; Her. 7.139 sed iubet ire deus echoes Aen. 6.461 me iussa deum, quae nunc... ire... cogunt; and Her. 7.186-90 fetishises Aeneas' sword even more than in Aen. 4 (though the use of a sword to commit suicide is actually a traditional part of the Dido story -- just not Aeneas').
And the seed of the pregnancy is there in Vergil too. Vergil's Dido wishes to be pregnant by Aeneas: see Aen. 4.328-9.
So I'd say yes, it's Ovid's own addition: he's taken Dido's wish to be pregnant in Aeneid 4 and made it that much more vivid, in an effort to depict Dido the woman as opposed to Dido the aristocratic dynast.