Thank you for asking the consultation.
Since you are having persistent cough, its wise to take an antibiotic to avoid any further complications. The antibiotic your Doctor has recommended is a Augmentin 625, which is actually a combination of Amoxicillin 500mg + Clavunate Potassium 125 mg (a medicine which helps the antibiotic amoxicillin to act better - not an antibiotic per se). So basically the antibiotic dose is 500mg, which is not high at all. In fact this is the required dose. If you take the medicine in a lower dose, it might not given any benefit. Both are Pregnancy category B, which means they are rather safe during pregnancy. And since you are in the 8th month of pregnancy drugs will not have a major effects on the baby as the organ development is complete.
Other alternative is to take Amoxicillin alone, 500 mg thrice daily. (MOX 500mg). This is also a acceptable option. But taking a lower dose is not recommended.
Hope this helps. Thank you.