Was Nola really pregnant during the time of her murder?

I think there are two possible answers to this.

1. No, she was not. This assumes the police were somewhat good at their jobs. They never discovered her pregnancy, it never showed on an autopsy report and, perhaps more tellingly, she never documented it in her diary (despite her extensive writing about everything else). On the basis of this evidence, there's a strong suggestion that she wasn't pregnant and was lying to get Chris to be with her.

2. Yes, she was, but it was irrelevant due to the overarching "theme" of the movie - luck. Think of how the movie ends. To take a line directly from the Wikipedia Page:

The film ends with Chloe giving birth to a baby boy named Terrence, and his uncle blessing him not with goodness but luck.

Throughout the story, luck comes into play. Chris got rid of all the jewellery into a river. Just by chance, a one in a million chance, one of the rings doesn't go into the river but instead lands on the pavement. This ring is then pocketed by a drug dealer who is later killed. Since the prime suspect of the police was a drug dealer of some sort, this fit their "picture" of the crime perfectly and they stop investigating, leaving him in the clear.

So that's the sceond way of looking at things. Nola was pregnant, but by pure luck the police never found out, never noticed the bit in the diary with it (assuming it was there), Chris got lucky in that the ring he dropped ended up brilliantly pinning the blame on someone else and he ended up luckily carrying on his normal life, with his little boy.

I don't think there is a right answer, so much as two open possibilities. It really just depends which you prefer.

Wilton is clever at crime design and he will do what is best for his own life. Since divorce with his wife is much better than jail, he will only commit the crime when the plot is perfect and the chance of failure is very low. This means he is almost sure the pregnacy thing (which could be a major bug) won't be a problem before he commit the crime. This means:

1. Maybe she is not pregnant and he somehow know that. But even she is not pregnant, he still have to get rid of her by the crime.

2. Maybe she is pregnant but somehow he can be very sure this will not be a problem, like he will first check whether the phase is detectable in local autopsy.