Is there a registery where biological parents, mothers can go to help their children find them?

There are several.

Adoption.com has probably the most prevalent one.

You should also register with the adoption agency that you placed through, as they will hold that information to give out if your child comes looking for you.

You can not, however, start a search until the child is 18. You can register with the agency earlier than that, but the information will not be given out until after the child's 18th birthday.

Check with ScottGem, he knows of sites also.

Adoption.com - Information on International, Domestic, Child & Agency Adoptions, Stories, Laws - (http://www.adoption.com)
Coming Soon... (http://www.soaringangels.com)
PeopleFinders.com | Internet People Search & Public Records (http://www.peoplefinders.com)
Free People Search by ZabaSearch! (http://www.zabasearch.com)
Yahoo! Groups - Join or create groups, clubs, forums & communities (http://www.yahoogroups.com) do a browser search for the state in which you were born into and the state that you were adopted into and learn the adoption laws in your given state. If the state has open records, then all it takes is a phone call. Reunion or not.
If the state has closed records, then it can take a lot of commitment and time to find your biological family.
If an adoption agency was used, you have a right to non identifying information. You also have a right to file a confidentiality release statement in your case file. If someone in your biological family inquires, a sister, a mother, a brother, a father, an aunt then all they would have to do is pick up the phone and call you. And then you and your family will take it from there. Some reunions are happy. Some aren't. Each individual family is different, just like the color's we choose for the house. Some are yellow. Some are sh brown. It's a gamble that most feel is worth the effort.