I am 34 weeks pregnant and my baby is already head down and the fundus measured 40cm is that normall?

Size > Dates: Having a vertex infant at 34 weeks is no problem at all. However taping out to 40 cm should prompt your OB to perform an ultrasound, determine a fetal weight, assess for excess fluid (polyhydramnios), uterine masses, and ensure normal growth & fetal development. A macrosomic infant (large baby) should prompt eval for diabetes, mode for delivery, & fetal monitoring if needed. Take good care.

No.: You have a size-dates discrepancy from your description. Your doctor/midwife should refer you for a maternal-fetal medicine ultrasound and consultation to determine what is causing this; it could be a fluke from excess maternal abdominal wall thickness, or true due to a large fetus, too much fluid in the pregnancy sac, uterine fibroids or fetal anomalies such as hydrops or sacrococcygeal teratoma.