Prior to ovulation, estrogens have positive feedback effects on both LH and FSH. The LH surge is usually mentioned in textbooks, but the FSH surge is often glossed over.
Going back to your original question about what stimulate estrogen production, LH and FSH each have their functions in this. FSH, as the name suggests (follicle stimulating hormone), helps promote follicle maturation. Since the follicle is the primary source of estrogens in females, FSH is contributing to estrogen production.
A few side notes. You'll notice I usually refer to estrogens in the plural. This is because there are several hormones that make up a class of steroids known as estrogens, there isn't actually a single hormone called "estrogen". The most predominant of these is estradiol, but there is some evidence there are biological functions of other estrogens even though they are present in much lower concentrations. Textbooks try to keep things simple and just refer to estrogen in the singular. The other thing worth noting is that despite the way textbooks refer to things as "known facts", there are a lot of things we don't know about reproductive endocrinology. (The same could be said for any field actually.) When you find yourself really puzzled about something in a textbook, and if you're really doing a good job thinking and understanding what you're reading you will find yourself puzzled by more than a few things, there's usually a much more complicated explanation behind it that either is very simplified for the basic text or the text is glossing over the fact that there are aspects of the system that really aren't understood well at all. When I teach introductory courses, I tell my students right up front that I'm going to lie to them a LOT while teaching the course. That's because you can't delve into everything all at once, so you have to simplify things and start out with rules before you can learn to break the rules. Sort of like when your grade school teacher tells you that you can never take the square root of a negative number, then in high school they teach you about imaginary numbers that allow you to take the square root of a negative number. My teaching philosophy is that by the time someone gets to college level, we can tell them we are sometimes oversimplifying things, and that really helps get past the confusing parts and allows the students to think for themselves rather than believing they are being told facts they should memorize and not question.
Anyway, given all that, do you mind if I ask why you are asking so many questions about reproductive endocrinology? It will help me know how much detail to go into with my explanations. If you're learning this for a class and are just trying to clarify some points in your textbook, I will try to stick to simple answers so you don't lose points for knowing too much (yes, it happens, if you know more than some people teach, you wind up losing points because you over think the questions on your exam and get confused when none of them is quite right). If you are just curious and want to learn more about the topic, I can get into more detailed answers. Or, if it's a combination of both (you're learning this for a class and have really gotten interested in the topic to learn more), then I can go into more detail but point out the simpler view that will be enough to get by on for an exam. See, it's just very easy for me to give general answers to topics I know only in general myself, but this is an area I know in great detail, so can easily overwhelm someone with too much information. On the other hand, I might make the error of being overly simple in my answers in attempting to not overwhelm you with too much detail. Just let me know if I err in either direction ;-)
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