Most of the major international medical insurance companies such as Goodhealth, William Russell, Interglobal, will cover the USA but the premiums will be considerably higher. Also there are restrictions on US citizens who return home so cover may only last for 6 months.
Also remember that all of the plans require that you be a member for at least 12 months before you can make a claim for maternity with some needing you to be a member for 12 months before conception so it is wise to plan ahead.