As with most infections, a person's metabolism will play a factor in determining how long it takes for the disease to progress. Some people will just succumb to it faster than others.
Since zombie infections typically attack the brain, the faster the virus can reach the brain, the faster it will take effect.
The amount of virus introduced could also have an impact. A trace amount introduced through a scratch would have to increase its population before it would reach major blood vessels, allowing it to really spread throughout the body, whereas a massive dose introduced via a deep wound would immediately be circulated.
This (somewhat) explains some of the discrepancies we see in duration of survival, and explains the relevance of the location of infection (i.e. a scratch on the foot or arm would likely result in a significantly slower rate of infection than a bite on the neck). However, given the general rates of infection we've seen, it seems almost certain that this would allow a variance of hours (or perhaps even minutes), and not days or even weeks.
This does seem to indicate that the pregnancy would have been the reason why the infection took so long.
The problem with this theory is that pregnancy actually lowers the mother's ability to combat most diseases and infections, particularly viral infections:
decreased activity of NK cells (?natural killer? white blood cells, which kill cells that have been infected with a virus or that are part of a tumor);
decreased activity of T cells, which help to control infections caused by viruses;
- and decreased production of cytokines (which are released from immune cells to recruit other cells to help fight infection)
The only way it seems likely that pregnancy would have helped slow the spread of the infection would be if it were a bacteria, rather than a virus:
- increased production of macrophages (cells that destroy bacteria). this improves antibody response and helps to protect you against bacteria-though they do not guarantee protection against bacterial infections;
The main problem with this is that the back cover of the DVD specifically identifies the source of the zombie epidemic as a virus (image obtained from a customer upload via Amazon.com):
When a mysterious virus turns people into mindless, flesh-eating zombies..."
However, according to the IMDB FAQ linked in the question, the writer, James Gunn, has denied the virus theory and has said that, when he wrote the screenplay, he envisioned it to be just the same as a vampire bite...more supernatural or inexplicable in nature. "Virus" is just a general term for something that can be transmitted into the body and do harm. It seems that the term "virus" was introduced by whoever was designing the DVD cover, and is not actually part of the film.
If it is indeed a "supernatural" agent, then hand-wavy explanations such as "pregnancy-related hormones" becomes much less of an issue, although I would instead lean towards the equally hand-wavy "the disease took longer because it was dealing with two lifeforces, instead of one, and focused on the weaker life force, first (the pre-born infant)".