I have had acne all of my adult life, for 12 years now. When I became pregnant with my twin boys my skin cleared up, almost immediately. It was amazing- and so, so nice. But, literally the day after I gave birth, I was greeted with at least 3 cysts. What does this say about my acne? Obviously, it's hormonal. I'm guessing it means I'm low on estrogen/high on testosterone? FYI, my acne started when I got the depo provera shot.
Same thing happened to me. I had full of acne as a teen... and it can followed me well into me 20's. I'm 24. (So much for hoping it would clear up after the teens..)
When I got pregnant, BOOM my skin was gorgeous. I've never felt more beautiful.. I've been breastfeeding for a year now and my skin is so, so bad. 
I know for me, when I had bloodwork done pre-preg I had high androgens.. the "male" hormone. So when I got pregnant, the pregnancy hormone progesterone took over and made the androgens inferior.. hence the clear skin.
Now on with my search to find what works for my skin. So far, nothing. bleh
My face cleared up so well during pregnancy and the part of breastfeeding when I wasn't ovulating. I'm SURE it's hormone related. The only thing I've found that helped me was minocycline (which I was on for years) and glycolic and salicylic acid at 20% strength. You can't buy it that strong in a store... but it's on amazon and ebay. It really helped clear my skin and my daughter's skin. We both stopped the antibiotics and use only the acid and BP. Our faces have never been this clear. No diet changes either.
I know multiple people who developed cystic acne during pregnancy when they didn't have it before. It obviously has to do with hormonal changes, but probably depends on the state of your hormones prior to pregnancy. That's basically how it goes for all hormones. For example, many people get clear with the birth control pill. Birth control (and I've tried many different varieties) always makes my skin covered in cysts.
My first ever post to this site! The same thing happened to me. I've had acne since I was 13--I just turned 38. The only time my skin has been clear in the last 25 years was when I was pregnant with my 2 daughters. My skin started clearing up right after I got pregnant, stayed completely clear (and no oily hair!) my whole pregnancy, and immediately crashed back to "normal" 6 weeks postpartum. Interestingly, it didn't change at all when I was pregnant with my son. I have had my hormones checked several times over the years, and they are always "in range". Of course, the range is so wide and is a one-size-fits all measurement, that it's practically meaningless. I have been told that I have poly cystic ovarian syndrome by a reproductive endocrinologist, despite the fact that I've never had facial hair, infertility, or been overweight. Apparently the diagnosis was made just on the basis of long term hormonal acne and a varying length menstrual cycle (average 37 days before children-- around 31 days after). In the past, I've tried everything- birth control pills of different kinds, Azelex, Differin, Minocylene, Retin A, a compounded topical of 15% benzoyl peroxide/4% erythromycin combo, and other things that I can't even remember. Vitamins, minerals, herbs. Dairy free, sugar free, etc. You name it, I've prob. tried it. The only thing I ever said NO to was Accutane. Not one of those things ever did a bit of good. I take that back-- the BP/Erthy. did help keep the symptoms in check, but certainly didn't address the root cause. Also bleached out a LOT of PJs & sheet sets using it over the years! Last year, I was prescribed spironolactone for the first time. For the first four months or so it worked great. I was convinced it was the miracle cure. And then, it just quit working. I have no idea why. Interestingly, the last time I had my hormones checked, my testosterone was low, and the dr. wanted to put me on some! I said no b/c I was scared to death that it would make the acne worse. I'm sorry I don't have any answers, but it might be worthwhile to have your hormones checked and see if anything stands out. Acne has so many different causes, and therefore cures, that trying to figure out your issues from someone else's history is sometimes just a frustrating waste of time. Trust me, after 25 years, I know!
I am 32, I've had acne since age 11, I still have horrible adult hormonal acne. The best skin of my life was during both of my pregnancies (both boys). My skin was clear, radiant, and I even went makeup free with confidence in the hospital. I have also tried everything with minimal to no success. I think my only cure is to fake my body/hormones into thinking I'm pregnant! Not likely to happen...