The German embassy should speed up the visa process in the case of pregnancy involving a child who will be a German citizen. Information about the right to have a visa before the birth of the child is detailed clearly in the visa handbook of the German Foreign Office (Auswärtiges Amt), which provides guidance to embassies on issuing visas.
Aufgrund der aufenthaltsrechtlichen Vorwirkungen von Art. 6 GG ist dem personensorgeberechtigten Elternteil bereits vor Geburt des Kindes ein Visum zu erteilen, wenn feststeht, dass das Kind mit der Geburt durch Abstammung von einem deutschen Staatsangehörigen oder durch Geburt in Deutschland von einer ausländischen Mutter, die sich bereits in Deutschland aufhält, ebenfalls die deutsche Staatsangehörigkeit erwerben wird. Der schwangeren Mutter ist ein Visum auszustellen (im Hinblick auf die Reisefähigkeit idR zwischen dem 4. und 7. Schwangerschaftsmonat), wenn die Geburt mit hinreichender Wahrscheinlichkeit zu erwarten ist.
In English via online translation:
Due to the residence law preliminary effects of Art. 6 GG is to grant a visa to the parent with custody before the birth of the child, if it is established that the child was born through descent from a German nationals or through birth in Germany from a foreign mother who already resides in Germany will also acquire German citizenship. A visa must be issued to the pregnant mother (with regard to the ability to travel, usually between the 4th and 7th month of pregnancy) if the birth takes place with sufficient probability is to be expected.
Here is the link to the handbook:
The relevant section is on page 288.
I encourage you to make contact with the German embassy and make reference to the expected delivery date and the section I referred to above.
I advise you to emphasise the pregnancy rather than the marriage when detailing your situation to them. Although the separation of a married couple is a difficult thing, in terms of the German immigration law it is seen as a reasonable hardship (in German ‘zumutbar’) to have to wait for regular visa processing times before being reunited. A pregnancy is regarded differently due to the necessity for parents to bond with the unborn child and newborn.
Here is a good article by a lawyer about such cases (in English via online translation):
If it is possible for you to come to Germany again on a visa-free 90 day basis, then an alternative is to come here and apply for a residence permit at the immigration office (Ausländerbehörde) where your husband resides. The immigration office will most likely at first tell you to leave Germany and return with the correct visa. However, due to the law around the protection of the family, they can make an exception to this requirement in such cases. I cannot, however, guarantee that they would and as is clear from the article I linked, it sometimes requires legal assistance to do this successfully.