Robitussin should be avoided during pregnancy. Benadryl can be taken. So you can take only Benadryl.
Please feel free for your follow up questions.
Robitussin has not been studied (due to ethical reasons) of the fetal effects so we do not know if there are adverse effects or not, so this is classified in pregnancy category C. Please read the resource;
Though, taken in 38 weeks may not be harmful. Robitussin DM and benadryl do not have known drug interaction.