I am 39 years old and 18 weeks along. I am on a minimed pump and I am still having to lower my basal rates. Has anyone not had a lot of insulin resistance? I heard that it can happen around 20 - 24 weeks. I am wondering if it will be drastic at first or if will come on slowly. Does every type 1 diabetic have this happen? I have had diabetes for 28 years.
In my experience (21 weeks through my 2nd pregnancy), there aren’t big insulin resistance changes in the first half of the pregnancy. A couple adjustments here or there, no biggie.
But as I got closer to my 3rd trimester last time, the changes became rapid! It’s not like all the sudden your insulin needs double, but they grow quickly. I had to make changes every week, sometimes a couple times a week. (Most of my changes were in I:Cs rather than basal.)
A bit of forewarning, you will likely also have LOWER insulin needs VERY late in pregnancy. I was fighting lows the last two weeks. (My OB believes this is because the placenta is breaking down, but my perinatologist believes it’s just because the baby is growing so fast that more of our glucose is being used to fuel the process.)
Everyone is unique, though. What I learned last time was to expect the unexpected! 
Mine started in week 19, was gradual at first for 2-3 weeks, then accelerated. There were a few weeks where I had to make big changes every few days.
Some women don't experience the lows, some don't experience the insulin resistance. So we are all a bit different.
I never experienced the 1st trimester lows, but my insulin needs didn't go up until I hit about 16-17 weeks, and then they went up gradually until around 24-25 weeks and then jumped all across the board; I;C ratios, basals, sensitivity, everything. It took a couple of weeks to get everything normal again, and since then it's been a steady climb. I'm 33 weeks now and I'm taking almost triple my pre-pregnancy daily amount of insulin. But I haven't had to make any changes in the last few weeks until yesterday. And that wasn't a major change...just afternoon basals.
During my first and part of my second trimester I had serious lows...this is my third and final pregnancy and I never experienced it with the first two. Now in my final trimester I am been experiencing resistance, doing a lot more insulin through the day and for each meal. Each pregnancy is different...but for the most-part it seems like starting weeks 20-24 you will have much more resistance.
I never had the crazy insulin resistance with my first pregnancy. Yes, I needed more insulin than before, but it was not more than double my pre-pregnancy needs. I'm currently 14 weeks along with #2. We'll see how this one goes :)