The research actually shows that there's no reason to think bp meds will delay onset or halt progression at all
- they're primarily used to lower maternal risk of stroke.
The reason for that is that hypertension is just a symptom. Controlling the symptom doesn't halt progression of the disease, which is driven by implantation issues likely caused by immune conflict between the maternal immune system and the foreign placenta. (Tolerating foreign organs is difficult, and researchers are working on the question of how it is that our bodies handle normal pregnancies - how do they negotiate with our immune systems to induce tolerance?)
Some women go into pregnancy as chronic hypertensives with quite high pressures, and for those women, controlling their underlying disease might provide a little benefit to prolongation of pregnancy, but no one's sure about that. Some researchers suspect that there are small benefits to a small number of people but the trials are too underpowered to detect that slight benefit. (For an example, that means that they'd need, say, 10,000 pregnancies in the trial to detect a statistically significant benefit to 100 of them by prolonging their pregnancies one week, and that 9,900 pregnancies would not benefit. I just made up those numbers, to give an idea of how while there might be a benefit, it certainly wouldn't help everybody, and there's little evidence now to expect it to be there.)