Is it true that low progesterone during pregnancy but not during regular (non-pregnant) cycles indicates a chromosomal abnormality?

This is my first pregnancy and my embryo is measuring almost 2 weeks behind what it should be based on my charts. My progesterone levels are pretty low (6) but my doctor says my HCG levels are normal so she’s not worried. I’ve never had a problem with low progesterone before and we conceived within 2 months because my cycles are very regular and hormones all looked normal at preconception appointment. Im thinking my body might just be getting rid of an unfit embryo. Could this be true?