If by 18-20 days past ovulation she has not shown heat, have her ultrasounded again to verify the pregnancy and to check for the presence of twins. If you wait too long, the whole pregnancy and your mare's reproductive year could be shot. Call the stallion owner to verify the pregnancy to them. To find out if your stallion is one of these, have a veterinarian who is knowledgable in equine theriogenology do a breeding soundness exam on your stallion either in the fall or in late winter. Have his semen tested in equitainers in different extenders.
Some stallions do very well in one brand and horrible in another.
His producing son may prefer the excact opposite that his sire does well in. Or, you can avoid bad PR due to the fact that your stallion in Maine can not make it to Texas without his sperm pooping out. It is strongly recommended that the mare owner take note here and discuss as many of these details with both the stallion owner and the professional who will be performing the actual insemination well in advance of the event. These are steps often forgotten and can lead to a nasty phone call.