A few months ago I wrote a short article for
iYogaLife.com about what poses to focus on while trying to conceive. I think this will give you some insight as to which poses to include and which poses to leave behind.
The one thing I did not include in the article is your question about twists. If you think you are already pregnant, I would back away from super deep twists like
jathara parivartanasana, pasasana and
marichyasana II and IV. You can still include twists in your practice that focus on the upper back, just try to not churn the lower abdomen.
"I'm trying to get pregnant. What poses should I focus on?"
You are already taking the right steps in preparing your body for pregnancy just by stepping on the mat. When trying to conceive I recommend a gentle flow practice with a focus on restorative poses.
You do not need to avoid any particular poses but do skip hot, vigorous, or dehydrating exercise. Focus on poses like Reclined Goddess Pose (Supta Baddhakonasana) or Uptavistakonasana (wide angle pose) because these two poses open and soften the belly and pelvis.
You can also include inversions like Shoulderstand (Sarvangasana), Plow and Supported Plow (Halasana) which gently compress the thyroid, an endocrine gland that helps regulate hormones in the body. The two hormones produced by the thyroid gland are called - t3 (triodothyronine) and t4 (thyroxine). This delicate balance of hormones not only effects fertility, but also effects menstruation.
Stress can also play a factor in fertility. Yoga has been proven to reduce the production of stress hormones which can hamper conception. I see great things happen to those who stay committed to their practice. Good Luck.