The fundal height is a less perfect marker of size. It measures not only baby, but also amniotic sac (so if you have oligohydramnios, you’ll measure small) and more importantly nowadays, abdominal wall fat. In bigger women it’s really difficult to glean any useful information from them.
The ultrasound is something to be taken more seriously. It not only estimates weight (using some sophisticated algorithms calculating the likely volume off of certain dimensions taken of the baby, and applying a formula estimating organ weight, bones, fluid, etc) but also crown-rump length, biparietal diameter, etc. As we keep saying, it’s definitely imperfect though. And if it’s off-- your baby is equally likely to be smaller as it is to be bigger than the estimated weight.
A baby who was growing normally for the first 8 months of pregnancy but levels off in the last month is alarming. It’s quite the opposite of the usual trend, where weight gain accelerates linearly for the baby. As you know, the most usual, and ominous, explanation is that something is restricting blood flow to the baby-- usually a placental problem, but occasionally the cord-- and not nourishing it well enough to permit that last growth spurt before birth.
This is a very complicated risk-benefit decision you’ll need to make with your midwifery team, and obstetrician if one becomes involved in your care. If you want to search the literature, the central clearinghouse for all peer-reviewed, verified research is www.pubmed.gov [this will be by far the most reliable way to research a topic, any topic, remotely related to medicine. Anything else should be viewed skeptically as opinion until proven otherwise]. The best possible evidence is a meta-analysis of several studies, preferably randomized controlled trials. This is where several studies are pooled together and looked at in common to see what the overall evidence really is. Think of it as a super-review article.
I found two meta-analyses of interest for you. The first is on fundal height measurements and how accurately they reflect genuine growth restriction for the fetus. Here is the paper:
It says that fundal height does NOT accurately predict problems, and that women with too-small fundal heights were NOT more likely to have low birthweight babies, dead babies, or admissions to the NICU, etc. [Again, this is likely due to what I was saying above-- it’s just a very imperfect, low-tech, variable marker].
The second paper is even more relevant. It’s a randomized controlled trial which asks the question, “what is the best, safest method to treat babies who are suspected of being too small? Should we follow them or induce labor?” This paper found no difference in the death rates of babies who were induced earlier versus followed expectantly, but babies who were born later had fewer NICU admissions (which makes sense, they’re older).
Here’s the original paper, again showing that there were no worse outcomes (death, severe distress, NICU admission) in babies who were expectantly managed versus induced.
Again, these inductions were random. Women were pre-allotted to the induce v. wait experimental arm with NO knowledge of their situations, so it’s not like the sicker babies were induced. It’s a true experiment.
On balance, it looks like the evidence supports watchful waiting, with weekly scans.