This is reported in around 5% of pregnancies at the 18-20 week scan. There is no need to worry if you are amongst this 5% – you can continue your daily activities as per normal unless you have been advised otherwise.
In your third trimester (until around 36 weeks) the bottom part of your uterus does most of it’s growing and stretching, taking the placenta with it. It doesn’t ‘migrate’ upwards, but being attached to the uterine wall, it’s carried upwards with it. In 0.5% of cases, the placenta doesn’t move up with the uterus – so given that small percent, there is an extremely good chance that your placenta will not be covering your cervix when it’s time to give birth, enabling you to have a vaginal birth.
If you have a low lying placenta at 18-20 weeks, this does not mean you need a caesarean section. The placenta will highly likely move as mentioned above. The uterus still has much growing to do, so an ultrasound late in your third trimester will give you and your carer a better picture on what’s really going on and if it really is of concern.
If you have Placenta Praevia, you will likely need to birth your baby by caesarean section depending on the grade or degree of Placenta Praevia. You may be able to birth vaginally if you have a grade 1 or 2 Placenta Praevia, however if you have grades 3 or 4, a caesarean section will be necessary. This wont be booked in immediately, if you don’t go into labour before hand, you will likely be booked in at around 38 weeks when your baby is more mature. If you go into labour prior to this you will need an emergency caesarean. A grade 4 Placenta Praevia will often mean being admitted to hospital at the first bleed and remaining as an inpatient until birth.
As for your babys position it is still quite early to start worrying to much as they still do move around a lot so just follow Optimal Fetal Positioning and also check out http://www.spinningbabies.com