The MAVEN navigation team successfully performed a period correction maneuver on Wednesday, August 12, which increased the orbital period of the spacecraft to 4 hours & 38 minutes. This maneuver was performed in order to keep the spacecraft within the required science corridor.
The maneuver was necessary because there is drag or an aerobraking effect exerted on the spacecraft during periapsis (lowest altitude), when MAVEN enters a more dense region of the Martian atmosphere, which can reduce the orbital period. This effect is especially pronounced during the deep-dip campaigns, where the spacecraft flies through a density corridor during periapsis that is twenty to thirty times the atmospheric density encountered during normal science operations. [More at link]