The pop star's new book, The Woman in Me, focuses in part on her 13-year conservatorship under her father James Spears.
Timberlake was "not happy" about Ms Spears being pregnant, according to extracts published in People magazine.
"He said we weren't ready to have a baby in our lives," Ms Spears said.
The pop star duo dated in their late teens between 1999 and 2002.
Mr Timberlake was "so sure that he didn't want to be a father", Ms Spears wrote in the memoir, which is set to be published on 24 October.
Ms Spears said she did not know if having the procedure was the right decision, and that she would not have had an abortion if the choice was "left up to me alone".
But, she added: "For me, it wasn't a tragedy. I loved Justin so much. I always expected us to have a family together one day. This would just be much earlier than I'd anticipated."
US ahead of the book’s publication on 24 October, Spears writes of the abortion: “For me, [the pregnancy] wasn’t a tragedy. I loved Justin so much. I always expected us to have a family together one day. This would just be much earlier than I’d anticipated. But Justin definitely wasn’t happy about the pregnancy. He said we weren’t ready to have a baby in our lives, that we were way too young … I don’t know if that was the right decision. If it had been left up to me alone, I never would have done it. And yet Justin was so sure that he didn’t want to be a father.” The couple dated between 1999 and 2002, when they were in their late teens and early 20s. Timberlake has not commented on Spears’ account.
Regarding the conservatorship, Spears writes: “Thirteen years went by with me feeling like a shadow of myself. I think back now on my father and his associates having control over my body and my money for that long and it makes me feel sick … I became a robot. But not just a robot – a sort of child-robot. I had been so infantilized that I was losing pieces of what made me feel like myself … The woman in me was pushed down for a long time. They wanted me to be wild onstage, the way they told me to be, and to be a robot the rest of the time.”
Jessica Biel and they share two children. Spears has also married and had two children. Both stars have also continued to grow their careers in entertainment. Still, it’s hard not to look back at their early years.  
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