I tried maternal milk, avocados and eggs! My baby was also almost under low birth weight. Came out at 2.85kg at 39+6. But if you can't eat much, maybe can try maternal milk as the calories and nutrients are more dense for the amount you consume.
I got frisomum which has about 400kcal for a 180ml serving. Don't buy the brand if you don't like vanilla flavour for milk, I personally added milo cause I didn't like the taste). There are other brands like Enfamama.
I'd like to say don't stress yourself over it though! I did and it wasn't helpful. I did gain a 1-2 kilos per week by the last few weeks and only a small fraction went to my baby. Babies can have healthy weight gain after they are delivered :)
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