Hi raviolig
I must admit I agree with your midwife as an anterior placenta means the placenta is positioned on the front wall of your uterus and a posterior placenta is positioned on the back wall of your uterus. The placental position can change from low lying (near the cervical os) to moving away from the os as the uterus grows , however as the place the placenta develops is wherever the fertilised egg embeds in the uterus it is unlikely to change from the front to the back of your uterus. I am unsure as to whether it may be different if you have a retroverted uterus when the uterus tilts backwards which then starts to tilt forward after 12 weeks changing the view of where the placenta is lying! However, you will need to discuss this with your obstetrician or the ultrasonographer at the next scan. Either position does not cause any harm to your growing baby however so please be reassured about that!
Dizziness and headaches can be due to low blood pressure especially in the first trimester, or even low blood sugars, so eat little and often and avoid getting up too quickly from a seated position. This is a very common symptom for many mums in the first trimester. The headaches can also be due to dehydration so increase your fluids and above all listen to your body and if you need to rest and lie down for a while do so!
Pins and needles in the hands can be an early sign of carpal tunnel syndrome in pregnancy so if it persists you should contact your midwife for advice.
Nikki