It could be, lots of women experience a wild swing in their libido when pregnant. Some become hot to trot and others go from hot to cold.

It could also be psychological; as some women, despite many assurances that sex during pregnancy causes no harm to the fetus, believe that they will somehow injure the pregnancy. The fetus is like an egg wrapped in a pillow surrounded by layers of foam padding, however, and if the pregnancy is without complications the developing child will not be harmed. He or she might, in fact, be soothed and calmed by the hormones released during orgasm, and the loving sounds of his/her parents.

Mama may also be feeling very overwhelmed by this life change and not interested in sex because of that. If this is the case then just gentle loving, even if it is from a distance, will help her to cope. Non-sexual touching and stroking will help her to release oxytocin which will ease her fears naturally and help her feel more connected to her body, growing baby and partner. There is much evidence that the reason women stroke their bellies when they are pregnant is to release these chemicals and sooth themselves. Also, I can say from experience, it feels good!

Whatever the reason, assuring her that it's ok and that she is still loved will ease the turmoil and leave her with the space needed to reach out and possibly resume sex earlier. Harshness or distancing/shaming behavior will make the problem worse, and can cause real damage to the relationship between she and her partner. This is a monumental life and body change, so be gentle with her and encourage her to be gentle with herself.