Britney Spears and Sam Asghari Back in L.A. After Birthday Fun

Britney Spears and Sam Asghari are finally back from Mexico after celebrating her big 4-0 birthday for nearly a week in Cabo ... and turns out their festivities included a third and fourth party!

Brit and her fiancé's private jet touched down Tuesday morning in Los Angeles, and they had quite the disem-barkment. Yes, the singer was holding one of her dogs, while an assistant carried the other off the plane ... and Sam trailed behind.

We didn't realize Lacey, a Maltese, and Hannah, the Yorkie, were with the couple during the birthday celebration. Good to see Brit kept the whole family together during the vacay.

As we previously reported ... the pop star's dogs were taken away from her briefly during the summer, because her housekeeper believed the pups were being neglected.