You asked

How important is it to be a healthy weight before getting pregnant?

For lots of reasons, it is a good idea to reach a healthy, ideal weight before you become pregnant.
Being underweight can reduce your chances of conceiving, and underweight women who become pregnant have an increased chance of delivering a low-birth weight baby. 
Low-birth weight babies are at more risk of developing heart disease when they grow up. So, doing your best to maintain a healthy weight and to gain a healthy amount during your pregnancy can help to programme your baby for long life and good health into their future.
Being overweight also carries increased risks during pregnancy. If you’re overweight you are more likely to develop high blood pressure or diabetes during pregnancy, and are even more at risk of a complicated delivery. 
So, for you and your baby, if you’re under or overweight before you become pregnant, now is the time to address the issue. It is also much easier to maintain a healthy weight during your pregnancy if you’re a healthy weight to begin with.
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More questions

Should I take iron at 30 weeks of pregnancy?
Every Mum-to-be is different and will gain a different amount of weight during pregnancy
For lots of reasons, it is a good idea to reach a healthy, ideal weight before you become pregnant – for both you and baby. 
You are now on the final stretch and it is an exciting time when you finally reach the final trimester - each day bringing you closer to meeting your new baby
Hopefully, if you did suffer from morning sickness, it will begin to pass now...
Congratulations! You are setting out on the first steps of a very exciting road, and no doubt looking forward to meeting your baby at the end of these 40 weeks of pregnancy.