unable to send parameters using http.request in node.js

The script that I am using is:

var http = require('http');
var options = {
  host: 'some url',
  port: 80,
  path: 'path',
  method: 'POST'
};

var req = http.request(options, function(res) {
  console.log('STATUS: ' + res.statusCode);
  console.log('HEADERS: ' + JSON.stringify(res.headers));
  res.setEncoding('utf8');
  res.on('data', function (chunk) {
    console.log('BODY: ' + chunk);
  });
});

req.on('error', function(e) {
  console.log('problem with request: ' + e.message);
});

// write data to request body
req.write('username=abcd&password=defg');
req.end();

I am getting a response as 404 bad request by server.. but when I use the firefox add-on "poster" for posting the same parameters to the server.. I am getting a proper reply, i.e 200 status code.. I think this the problem with the string I am passing in req.write(). but the same string is working fine with poster.. Can someone explain what the issue is?