lilies: since January you can always apply with a private German health insurance
for the "Basis" tarif which is the tarif that is not allowed to ask for pre-existing health conditions (or pregnancy for that matter). So, yes, you can get coverage - BUT: this tarif will be quite expensive, probably around 400-500 EUR/month. It will cover all the approved check-ups during pregnancy as well as childbirth in hospital and of course treatment afterwards. Since even a "normal" pregnancy and childbirth will run up a total bill of 6-7 thousand EUR, you would still be better of with this than without. Even more so, if there would be any complications during the pregnancy or birth.
a normal private health insurance will not accept you once the pregnancy has been determined. Are you only assuming to be pregnant or have you already had a check with your doctor/gynaecologist? There might be some leeway if you have not left a papertrail at German doctors yet...