I would tell them, especially if you plan to see one another over the holidays. Better that then have someone figure it out and get stuck with an awkward confrontation.
Even the most religious families these days are pretty savvy about the probability that your relationship is already sexual, and you are adults, so if you aren't in the habit of setting boundaries, now's the time. You know there are going to be questions about your relationship, so why not get together with your boyfriend now and plan for how you're going to answer them? Think about it: why are you back together/why did you break up? Are those issues resolved? Do you plan to live together? Get married? Will his name be on the birth certificate? And so forth. Have some answers ready, show them how thoughtful and mature you are, and it'll go a lot more smoothly.
i agree i got pregnant with my husbands baby the same month i turned 18 note my husband is 35yrs old, ha. my mother hates his guts but loves the baby and she keeps it at a "getting along" level for my daughter.
(p.s. i told her over the phone. your parent will be mad at first thats why its best to tell them early so it can sink in and anger will pacify by the time of labor.)
I know it's hard, but just tell them. No matter how much they don't like your boyfriend, they will love their grandchild. Also it might be beneficial to tell them why you broke up, with out pointing fingers. Ie "someone told me things about him that we found were untrue." Ultimately they love you and are looking out for you. Remember no matter how harsh they may be they are coming from a place of love for you.