The first 26 weeks of maternity leave are known as ordinary maternity leave (OML) and the second 26 weeks are known as additional maternity leave (AML).
Additional maternity leave starts on the day after the ordinary maternity leave period finishes.
One important difference between ordinary and additional maternity leave is the extent of your right to return to work at the end of your maternity leave.
At the end of OML, you have the right to return to your old job without change or prejudice. At the end of AML your right to return is different. You still have the right to return to your old job, but if it is not reasonably practicable for the employer to offer this back to you, you can be offered an appropriately similar role on no less favourable terms. Things like seniority and pension conditions must remain unaffected.