Have you tried the journal "Pregnancy Hypertension" ? It is the journal of the International Society for the Study of Hypertension in Pregnancy (ISSHP).
Data from the birth certificate in the US contain both pre-pregnancy and gestational hypertension as well as eclampsia. However the incidence is underreported on the birth certificate. See the 2013 final birth report under my publications for the most recent data.
Yes, and to add to Jaime's response, you could run a key word search (using key words like preeclampsia, eclampsia, and so on to search relevant databases and websites. You could search Pubmed, Mediscape, and open access journal sites for resources that generally address the issue. You could also look through the reference lists of articles on the condition and from there you could identify some sources that could be useful to you. Of course a most fruitful way would be to first decide which area of hypertension in pregnancy you are particularly interested in; so the point raised by Jaime cannot be over emphasized.
Extensive studies about all kind of hypertensive disorders in pregnancy are done by Prof. Nicolaides at Kings college in London. His work group focuses on screening programmes for these diseases. So if you search on pubmed for pre-eclampsia and Nicolaides KH, I am sure you will find what you need.
One of the best ways to keep up with current science is to set up a Google Scholar alert for pregnancy and hypertension. It is free, and it searches a wide breadth of the literature. Go to http://scholar.google.com/ and look click on "Alerts" on the top of the page, then follow instructions to get regular updates in your email account.