If I were you, I'd have my radar tuned up a little, but I wouldn't be really stressed or worried yet (I know it's hard). Reason being, you've got some positive things going for you right now.
First off, 39th percentile is great! That's just a hair left of normal, and at this stage, it's likely to change a lot. Babies do have growth spurts, and ultrasound is subject to error. With my current pregnancy, I went from 82% to 52% to 32% at 24, 28, and 32 weeks, then it rebounded back up in the 50s and now it's settled into the 40s. And honestly, I think any neonatologist would agree that growth doesn't help their fight as much as development does. My little man was born at 27 weeks, but was the size of a 22 weeker, or below the 0 percentile. He had some touchy beginnings, but did very well. In the NICU, every specialist that saw him would look at his birth weight first (470 grams), get a confused look, and then revert back to his birth gestation for risk assessment because development occurs at gestation milestones and not necessarily with growth. He's now 3 and completely normal in every way, except he's really tall, like above the 90th percentile. Go figure!
Secondly, congrats on 30 weeks! You are now leaving the really really scary NICU stuff (typically). Get through the next few weeks and you should feel even better. My nephew was born at 34 weeks & 0 days, and he only spent 10 days in the NICU, mainly just learning to eat while they watched his temp and breathing (which was very good).
And lastly, do you know what your amniotic fluid level is? An AFI of 5 is usually cause for delivery. It might just be on the low side of normal, and yes this is a placental function, which can correlate to pre-e. It's something that should be watched closely, but it doesn't necessarily mean that you're going to get pre-e. I've had low-normal levels for 3 weeks now (my AFI=7 to 8), and it's been pretty stable. I also drink a lot of water, so I was surprised by the dip. I rest more now, not bedrest, just no crazy errand or shopping days. Has your OB looked at cord flow? 4 things to watch vigilantly: your BP, cord flow, amniotic fluid level, and growth.
Hang in there!
Miscarriage Feb 05 & Dec 07
Daniel born Feb 09 at 27w5d, 1 lb 1 oz/12 inches (severe IUGR) due to Severe Pre-E & HELLP. Now tall for age and no complications.
Diagnosed with MTHFR
Cornual Ectopic miscarriage Mar 11
Natalie born 6-5-12 at full term, 7 lb 11 oz & 21.5 inches, progesterone 2-8 weeks (for suspected luteal phase defect), lovenox 6-29 weeks, lda 0-36 weeks, folic.