Bring proof that you will return to Canada. School transcripts showing you are enrolled and have paid for the upcoming semester are a start. How about car payments, letters of support from whomever you are staying with, hospital appointments for your new born which need to attend.
All of the above are good things to bring, I am sure more people will be a long with more suggestions for evidence.
The first two answerers gave you sound advice. You shouldn't expect not to be referred to secondary, and you shouldn't be surprised that the border officers suspect that you might be pretending to be a visitor but have the true intent to stay forever.
After all, the U.S. has about 20 million illegal aliens many of which said at entry that they were "just visiting." Not every illegal alien sneaks into the country on foot without inspection. 50% of the illegals arrived with visas, and as a Canadian, you're exempt the visa requirement so the only government official who has a chance to evaluate you is the border officer.
The best way to avoid being sent to secondary at the border is not to cross the bridge. If you want to enter a country other than your own, you have to submit to whatever inspection the border officers deem necessary. As you have not been refused admission, it appears that you have convinced them in the past that you're legitimate.
Stick with what has been working. Perhaps eventually you'll become even more convincing by establishing a record of honestly and compliance.
Nothing you bring is going to guarantee you are going to go thru Customs without additional questions and pulled in for secondary processing. I would bet you will continue to get pulled into secondary because your situation raises the red flags that require the additional questions to be asked. Best bet is just be prepared and yes, that means prepared to return to Canada and not allowed into the US.
As the other poster said - bring your papers from the University saying that you are registered and paid for the upcomming semester. Bring a letter from your parents that you live with stating that you and the baby live with them and help you with housing and support you while you are attending school and that they fully expect you to return from the US after your visit. Have you and your boyfriend thought of the future? What are your plans after your University? Get informed of your immigration options either you to the US or him to Canada and do your research. Be informed so that you can tell the border officer that "We plan to after I'm done University in Canada to ....blah blah get a Fiance visa to the US..... blah blah....or get married and him immigrate to Canada... blah blah..." This will at least give the impression that you have your head on and intend to comply with the regulations and aren't a risk of staying in the US illegally.
Whatever you do be honest..
Good luck to you.