Just to let you know, if you NEVER feel that urge, you're okay and perfectly normal too -- you're childfree. There are now 1 in 5 women in developed countries today who choose never to have children and who just aren't interested.
Basically, your 20s are the best time, or as good a time as any. Being ready to have a baby is so very personal and everyone is different. There are some people like me who never felt any urge to do it, to women who also just knew when the time is right.
Be prepared, though, because it's hard and tough and an uphill climb. I haven't had one, but I've kept my eyes and ears open all my life, and I'm a teacher.
You're far better off with at the very minimum a committed partner and best off with a husband, although nothing ensures that the man will stay around to help raise the child. Evolutionary history usually tells us that men stay around an average of 3-5 years to ensure that the child survives infancy, and then the males of most species leave. But of course humans are different, and if you are married to your soulmate, then you'll find a way to stay together, in spite of having a child.
I say "in spite of" quite deliberately, because you won't have that same level of happiness that you had before a child -- it'll just be different. But there will never again be significant "just us" time for about 20 years. If you do get "just us" time, you will have to carve it out, make time for it, plan it and fight to keep your "just us" time.
I agree with almost everyone here: just because you may be feeling the biological urge to have a child doesn't mean you should immediately. Give yourself a few years with your husband, because once a child comes, you will never have the same closeness again, or it will be a couple of decades until you do. Travel, explore, and most of all think about what you really want. I echo what another poster said and think of adopting. After all, we are 6,500,000,000 (billion) people now.
Just remember, having a kid is a crapshoot. Right now you cannot predict what kind of health your child will have. Think seriously and talk to both sets of parents before you begin trying to conceive. I personally could not imagine wanting a child KNOWING that it could have a strong chance of inheriting something really nasty, like a heart, musculo-skeletal or personality disorder. What kind of life is that? I think it's better to forgo having a child if there is significant disease present on one or both family trees.
Ultimately, though, having a child isn't always so rational. Our genes want us to procreate, even when doing so isn't always the best or wisest course of action. But we are human, and we do have the capacity to think and plan and strategize. We can do what makes sense for ourselves and for any life we bring into the world.
No matter what you decide, it's a very good thing to think about what you really want for your life. You're a better and more thoughtful person for considering the options; so many people just up and decide they want a baby and get pregnant without thinking about their future, their finances, or their living situation or their family health. Our society is certainly pro-natalist. You'll never be dissuaded from having a baby.
Ultimately you have to think: all the crappy diapers, the 2am feedings, lack of sleep, redistribution of money, extra expenses, possible genetic abnormalities, crap, pee, screaming and tantrums every hour of every day of every week of every month for SEVERAL YEARS -- is it worth it?
Yes, there are a few Kodak moments... but that child also has to sh-it and piss and throw up and test your limits. Only you know if it's worth it or not. But don't, whatever you do, please DO NOT romanticize pregnancy or parenting -- it's a long, hard slog filled every inch with challenges and tests and trials. Not worth it to me.
If you do decide to have a baby, talk extensively with your partner/husband. Decide if you only want one child or more. Decide jointly on how to discipline, both verbally and corporally. Talk about what you would do if your child were developmentally delayed... or extremely intellectually gifted. Both present unique challenges for parents and children.
Now if you'll pardon me, I'm going to go plan my next European vacation, check my stocks, go work out, and ask my husband when he wants sex. We don't have kids, by choice, and that 'road less traveled' is pretty sweet. I'm glad to be a teacher: happy to help teens, because I do like older, more independent kids, and because I don't like younger kids, and even happier to send them home to their parents at the end of the day! ;)
Glad you are thinking about it. Cheers, K