Did the NICE guideline for progesterone treatment of threatened miscarriage get it right?

In November 2021, new guidelines were published about the management of bleeding in early pregnancy. If someone who has had a previous miscarriage starts bleeding, they should now be treated with progesterone as this slightly reduces the chance of miscarriage. The guideline says progesterone should be given if the pregnancy is in the womb, and potentially normal, until 16 weeks of pregnancy. However, in the big studies looking at progesterone's effect in reducing miscarriage the beneficial effects of progesterone were complete by 12 weeks of pregnancy. At that stage, it is the placenta and not the mother's ovary that makes the progesterone to support the pregnancy. We do not know the long-term effects of giving extra progesterone during pregnancy on the offspring. Some research has raised the possibility that there might be some adverse effects if progesterone is given for too long. Maybe the guidance should have suggested stopping at 12 weeks rather than 16 weeks of pregnancy.