Complex sentence: Migration from new EU member states in Central and Eastern Europe since 2004 has resulted in growth in these population groups but,, the trend is reversing and many of these migrants are returning home, leaving the size of these groups unknown.
Split into simpler sentences: Migration from new EU member states in Central and Eastern Europe since 2004 has resulted in growth in these population groups but,, the trend is reversing. Many of these migrants are returning to their home countries, leaving the size of these groups unknown.