The couple, based in Geneva, were earlier sentenced to life for strangling a 20-year-old woman, identified as Loreidy.
They had attempted to cover their tracks by burning the body and burying it in a wooded area in the canton of Fribourg.
The case, involving a love triangle, dates back to November 2007.
But last year Switzerland’s supreme court ordered a review of the life sentence against the 30-year-old man and his 32-year-old girlfriend, saying the term was not justified by the arguments presented.
The appeal court ruled that the couple’s actions were “exceptionally serious,” the Tribune de Genève reported on its website.
“They killed with a noticeable lack of scruples,” presiding judge Pierre Marquis is quoted as saying by the newspaper.
The appeal court found no extenuating circumstances in the crime committed and they dismissed a defence argument about the convicted man’s troubled background (he moved to Switzerland from Angola when he was nine).
But the judges took into account the couple’s relatively young age and the efforts they were making to pursue studies while in prison, the Tribune reported.
The couple had a “partial awareness” of the gravity of their acts and therapy they received in jail appeared to have a positive effect on them, the court argued.
With its sensational details, the case has been widely followed across Switzerland.
According to past court testimony, the 30-year-old convicted man made love in a car to the victim he had made pregnant before he and his girlfriend slowly strangled her.
The girlfriend, who already had a child with the man, was jealous of Loreidy and wanted her to have an abortion.
After she refused the couple concocted a plan to murder her.
Because of the quirks of Swiss law the new sentence appears to have only a minor impact on the amount of time the couple will actually spend behind bars.
Under a life sentence they would be eligible for conditional release after 15 years, Geneva prosecutor Yves Bertossa is quoted as saying by the Tribune de Genève.
With a 20-year term they are eligible for such a release after 13 years.