OGR/GDAL Create Shapefile lost some parameter Spatial Refer

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import os
from osgeo import ogr, osr

ogr.RegisterAll()

# Shapefile lost parameter +towgs84
pathFile = '/home/lmotta/data/script_gis_dev/data/test_utm_sad69.shp'
strDrive= 'ESRI Shapefile'

# GML use all parameters
#pathFile = '/home/lmotta/data/script_gis_dev/data/test_utm_sad69.gml'
#strDrive= 'GML'

proj4Sr = '+proj=utm +zone=21 +south +ellps=aust_SA +towgs84=-57,1,-41,0,0,0,0 +units=m +no_defs'
sr = osr.SpatialReference()
sr.ImportFromProj4( proj4Sr  )

# Create Shapefile with SpatialReference (delete if exist)
drv = ogr.GetDriverByName(strDrive)
ds = drv.CreateDataSource(pathFile)
lyr = ds.CreateLayer(pathFile, sr, ogr.wkbPolygon)

print 'proj4Sr: %s' % proj4Sr
print 'Layer  : %s' % lyr.GetSpatialRef().ExportToProj4()

# Free
ds.Destroy()
ds = None
lyr = None
if strDrive == 'GML':
  os.remove(pathFile)
  os.remove(pathFile.replace('.gml', '.xsd'))
else:
  drv.DeleteDataSource(pathFile)