You really need to talk to your doctor. Do not stop taking your medication until you talk to your doctor. Your doctor can help you figure out a safe way to have your pregnancy. Until you do talk to a doctor, here is some general info:

The mayo clinic says:
"If you have ulcerative colitis, talk to your doctor before becoming
pregnant or fathering a child. Some medications used to treat IBD have
the potential to cause birth defects or can be passed to the baby
through breast milk. Active ulcerative colitis increases the risk of
fetal death or preterm labor. If you're already pregnant, be sure
you're cared for by a doctor who has experience with IBD and pregnancy."

A study done on 97 pregnant women with ulcerative colitis that were studied over a 12-year period found (quoted directly from http://www.ncbi.nlm.nih.gov/pubmed/6669937 ):

• For a woman with ulcerative colitis the risk of an exacerbation of the
  bowel disease was 32% per year in her fertile years, whereas it was 34% per year during pregnancy. This difference is not statistically
  significant.
• As compared with women with an inactive bowel disease, women in whom the disease was active at the start of pregnancy had a small but
  significantly greater risk of spontaneous abortion and premature
  delivery. The frequency of malformations, prematurity, and neonatal
  hyperbilirubinaemia was not higher in the children of ulcerative
  colitis mothers than in those of healthy mothers.
• Birth length and weight of the children of mothers with ulcerative colitis equaled those for children of healthy mothers.
• In conclusion, pregnancy does not necessitate any change in the usual
  medical treatment of ulcerative colitis.
  
• Women with ulcerative colitis should be advised preferably to conceive at a time when their bowel disease is inactive.
  
• Generally, ulcerative colitis constitutes no indication for induced abortion.

You can also check out this helpful PowerPoint: www.ddfindia.org/3rcdhruu/anoopsaraya.ppt It has tons of info, here is some about medication:

Medication Safe to Use During Pregnancy:

1. Oral, topical mesalamine
2. Sulfasalazine
3. Steroids
4. TPN
5. Loperamide*

Medication with Limited Data

1. Olsalazine
2. Azathioprine
3. 6 MP
4. Cyclosporine*
5. Metronidazole*
6. Ciprofloxacin*
7. Infliximab

Contraindicated (increases risk)

• Methotrexate*
• Thalidomide*
• Diphenoxylate*

*means to avoid when breastfeeding.
I googled your medication and found out Imuran is Azathioprine, which means there is limited data regarding the risk to the fetus (http://www.medicinenet.com/azathioprine/article.htm ). Pentasa is mesalamine, which is possibly safe (http://www.drugs.com/pentasa.html ). And Prednisone is a steroid which is also on the safe list (http://www.drugs.com/prednisone.html ).

Keep in mind I am not a doctor and the best thing for you to do is to make an appointment with your doctor. Good luck!
 

I'm not a medical anything, but apparently a lot of people are intolerant (or even allergic) to gluten, but don't realize it, and exhibit many varied symptoms that are often interpreted as something else. Have you been tested for sensitivity to various foods?

In any case, it sounds like a VERY risky idea to conceive when taking ANY artificial drugs (and probably even most natural ones), because man-made chemistry has had the least amount of long-term testing in all that exists around us, and mankind has a long historical record of crazy side-effects from many drugs, despite our "understanding" of biochemistry.