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Well, speaking as a husband, I think it's important to share all the critical things. Long before he becomes your husband, he should know the number of partners you've had, whether you've been pregnant, any major issues you've had with your family. Why? Because your physical and emotional health are not only important to you - they're important to him and will have a bearing on your life with him and any children you will have together, for as long as you live.
The number of partners you've had, and the seriousness of the relationships, will give him a better picture of what to expect from your relationship (people with tons of breakups may have trust issues, for example), and of course he needs to know whether you both should be double-checked for STDs. Aborted pregnancies can affect your ability to bear children in the future, so he needs to know that, especially if you're planning to have kids, and it can also have long-term (and potentially deeply-buried) psychological effects (guilt, depression) that might not manifest themselves for some time.
Family issues are important to communicate as well - any abuse, again, can bear on the mental health of your family, and he should be willing to help you through counseling. Mental health issues can also be deeply-buried but can affect your long-term relationship, and some may be passed on to your children genetically.
And this probably goes without saying, but you should absolutely open up to him about your hopes, dreams, fears, etc. You should work together to build a picture of your future, and full understanding of each other is critical to ensure that the marraige will last for the long haul.
All that said, you can certainly draw the line at details. He doesn't need to know the names of all your past boyfriends - though if he asks, it's not something that should be hidden (because of course it will plant a seed of doubt in his mind - wondering why you're withholding information from him, and whether there's something you're trying to hide.
I hope that helps. I've been 100% open and honest with my wife, she's been the same with me, and we're going strong after almost 15 years of marriage.
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- Wow lots of answers, there was a lot of answers I liked but I can only pick one. This answer was very long and I can tell you put your time and energy into it. Thanks =)