What causes a loss of sex drive in pregnancy?

Different factors. There are different factors. Psychological, hormonal, situational. Indeed some women experience an increase sexual drive. Each body reacts differently to same stress factors.
Fatigue. There are lots of reasons. Some women actually experience an increase in their sex drive. I think the fatigue that normally accompanies pregnancy contributes to the loss of desire.

Related Questions

Is low sex drive normal during pregnancy? This is severely hurting my relationship for him and me. Very stressful

Lots of changes. Hormones fluctuate widely during pregnancy. Some women find their libido has greatly increased, where others it greatly decreases. This can also happen if you have severe "morning sickness" - it's kind of hard to be in the mood if you're feeling nauseated and throwing up all the time. Long and the short, it takes an adjustment. Take more time with each other, more romance. Good luck!

Can stress be the cause of me feeling tired, losing interest of things I had a lot of interest in, low sex drive, sadness, and loss of motivation?

SPEAK TO SOMEONE...D. You sound depressed to me. You could speak to a good friend or pschologist to begin. Stress can add a lot of emotional turmoil too.
Stress & depression. You list here several of the main symptoms of major depressive disorder. Life stresses can certainly trigger depressive episodes -- but you also need to see your primary care doctor to investigate for any other illnesses that can look like depression, but aren't. Depression is certainly treatable with a combination of psychotherapy and possibly medications if necessary. Good luck to you.