Do I need an attorney to guarantee that my adjustment of status is successful?

Best wishes for your recent marriage and your pregnancy! It indeed is possible and lawful for people to file adjustment of status applications on their own, without assistance or representation from an immigration lawyer, and since many details about the immigration application process involve the subjective discretion of a USCIS adjudicating officer, no immigration attorney ever can guarantee the outcome of a case (and it may be ethically improper for an attorney to offer such a guarantee). Nonetheless, the likelihood of success is significantly greater when a couple is represented by counsel, and that may be particularly true where, as in your case, you may need to overcome a presumption of visa fraud through entering the U.S. on the visa waiver program and then so promptly becoming married (use of the Visa Waiver Program is only for people who enter with the intention of staying temporarily and then departing within the specified time). With prenatal expenses, it may seem challenging to afford to engage an immigration lawyer, but the cost for legal representation may be far less than you are anticipating (and far less than the cost of trying to resolve problems that could be created through denial of an inappropriately prepared and documented marriage-based adjustment of status case. Especially if being able to remain in the U.S. is very important to you and your family, it would be wise to engage an immigration attorney to assure that your case is properly handled from the very beginning. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.