Yes, of course, you can manifest after having sex. It's never too late to start with the mindset work. In fact, you are constantly manifesting; your subconscious works 24/7 - this is all about getting in control of what you manifest. But don't forget that this is not some magic trick, so if you haven't conceived, you can't just turn that around by using some pregnancy affirmations. You also can't put a negative pregnancy under your pillow and wake up to a positive one.