Should i believe him after he's lied to me this many times?

Well I am 8 1/2 months pregnant and have been with this guy almost a year and four months. We used to smoke weed together all the time and party and we both smoked cigarettes. When I found out I was pregnant, that all stopped. I immediately quit...

everything. My boyfriend had gotten in trouble around the time we found out I was pregnant and was placed on probation, for 5 years. Because of that, he quit drinking and smoking weed, but continued with the cigarettes. The smell of cigarettes bothered me so much I asked him to quit for me, and most importantly, his unborn son. He told me that he had quit and about a month went by when his little brother told me he was still smoking. My boyfriend admitted to it, said he was sorry, and promised he would quit and not lie to me again. A few weeks later he was sentenced to one weekend in jail and when he got out I was on the computer and smelled smoke, so I followed the smell and it led directly to the bathroom where he said he was taking a shower but instead, he was sitting on the toilet smoking a cigarette. I tried to leave him but once again, he promised me it would never happen again and said he was really sorry. A few more weeks went by when we got into a huge argument (it had nothing to do with cigarettes) and I told him I just wanted the night alone to think about things, and made him promise me he would not drink, or smoke anything of any kind. He promised. A couple days later his own friend told me that the night I went home my boyfriend was drinking. So, I tried to leave my boyfriend and he promised me it wouldn't happen again if I stayed with him, so I did. A while went by and we had no problems, he even asked me to marry him! I said yes. Now, about a week ago I thought I would come to his house to surprise him when he got home from work. In his room, I found a cigarette butt in an empty tea container. He swears it's not his and says that someone must have went in there and smoked it. But his door is locked while he is at work. So nobody could get it but me and him since we have the keys. The next day I got his jacket from his car and there was a bunch of tobacco coming out of his pocket. I questioned him, and he said he never put anything in there so he didn't know how it got there. He even tried telling me it wasn't even tobacco. I told him to just tell me the truth and I wouldn't be mad, but he still denied it. Today, I finally told him, you tell me the truth, or that's it. I'm done for good. He told me that he had put a cigarette in his coat pocket but it broke and that's why there was tobacco in the pocket. He still denies the cigarette butt in his room and swears it was someone else who put it there. Every time he lies to me I get extremely upset because obviously it hurts and he just keeps doing it. He told me again today for what must be the 5th time, that he promises it'll never happen again and he'll never lie to me again. Should I believe him after he's lied to me this many times? And if it was you, what would you do? Almost 9 months pregnant, and your fiance just wont stop lying. I don't know what to do, I love him but I am terrified I am going to get hurt again.