Congratulations!
You Mum will react how she reacts, and if she reacts like she has done in the past, then that's frankly not a Christian attitude at all and it should be pointed out to her* - maybe not by you, but by friends, other family members and her religious leader, whoever they are...
It is your relationship with your child, DP and God that matters. Talk it through with your religious leader if you feel the need and become secure that the decisions you've made are NOT based on the twisted and unchristian views of your mother.
Definitely don't get married in a hurry just because you're pregnant. If you want to be married, and if you're worried about getting through 20wks, then plan the wedding, but for after baby is born. I know someone that had a beautiful religious ceremony combined with the christening of their first born...and having done it that way, they felt so relieved. None of the tension, she felt great and had the joy of her by then accepting families around her to celebrate both milestones in her life.
In you gut you don't want to marry "just because you're pregnant" and you want to know that baby's here and your happy family's stable. God will not mind this at all and will accept that this is what is best for you and your family.
Alternatively, if you do decide to marry quickly, as I did, not for any particular reason, other than having the money to have what we wanted, we managed planning a civil ceremony for 20 people and lunch, and a meal in the evening with friends, plus suggested accomodation, flowers, nice ivory 2-piece outfit for me, suit for DH and Mums, registrar, going through the small hurdle of marrying in another district etc. all in 5 weeks...it was exactly what we wanted (limited fuss, just us immediate family and close friends) and we have no regrets in organising it so quickly. So, you could get to 20wks discover all is well with LO and then plan..! Though I do understand you may be somewhat more restricted with the religious element of your ceremony, but you could always talk about it with the church and see what they can do.
*Having watched a family member go through this, only with no devoted DP (one that threatened to harm her/baby in fact), her everso "christian" family all but threw her out...in that they told her she had to leave (at 19) and that she'd sinned and god would probably forgive her, but they weren't sure they could...bloody terrible human beings! Ultimatly though, they became utterly doting grandparents, but I for one have a lot less respect for them and do not count them as christian by any standard. I have no idea how she could do the ultimatly Christian thing and forgive them for the hurt they caused her.