Rhonda Patrick's instagram is always a good stop in the morning:

A new study that came out recently suggests something that shouldn’t be too surprising: that what you eat or possibly even supplement can, for better or worse, affect what kind of advantage you give your offspring. In this case, the study was looking at the effects of just a multi-vitamin, but studies have shown things like exercise can impact epigenetic programming too. There’s some evidence that nutrient status even a few months before getting pregnant may be important as well.

The study…
A randomized controlled trial with a 14-year follow-up found that pregnant women given a micronutrient multivitamin supplement starting in the first trimester had children with a 2.16-point higher IQ and 4.29-point higher verbal comprehension index compared to those in the control group (folic acid only). The intelligence quotient (IQ) for children from the maternal micronutrient supplementation group was equivalent to the increase in scores for approximately half a year of school.

The multivitamin in the study contained the following:
30 mg iron, 400 µg folate, 15.0 mg zinc, 2.0 mg copper, 65.0 µg selenium, 150.0 µg iodine, 800.0 µg vitamin A, 1.4 mg vitamin B-1 (thiamine), 1.4 mg vitamin B-2 (riboflavin), 1.9 mg vitamin B-6, 2.6 µg vitamin B-12, 5.0 µg vitamin D, 70.0 mg vitamin C, 10.0 mg vitamin E, and 18.0 mg niacin.

Since I know people will ask: I took the Thorne brand prenatal. I tend to buy a lot of their products because I think they’re a pretty good brand, but have not compared that to the multivitamin used in this study and have no real way of knowing if Thorne happens to be the perfect prenatal. I think it will be a very interesting future in which science helps further pin down how to create the ideal environment for fetal and early childhood development. This is obviously a very critical stage that sets the stage for many things (behavioral and otherwise) down the road.

Association of Antenatal Micronutrient Supplementation With Adolescent Intellectual Development in Rural Western China 7-9-2018
14-Year Follow-up From a Randomized Clinical Trial

The whole idea of supplements implies a deficient diet.

It's explicit in a few instances like fortified wheat flour and milk for example. Certain dietary ideas are based on de facto deficiency, like bulletproof coffee (add butter) which is said to be beneficial, but in fact can only be beneficial when there isn't enough fat in the diet to begin with. The late Seth Roberts experimented with supplementing with various fats and their effects on his cognitive abilities, he found that certain fats had a positive effect, while others didn't.

A secondary idea of supplements comes from implied poisoning of various bad foods like sugar and processed foods, as a sort of compensation mechanism to balance things out. Another idea of supplements comes straight from yet another idea about diet - a balanced diet. When such a diet isn't as balanced as it should be, supplements bridge the perceived gap.

Finally, supplements can be seen as therapeutic. We have ample experimental and scientific evidence for this, with experiments to treat and cure various medical conditions using high doses of various essentials. Personally, that's how I see supplements and my personal experience gives me ample proof for that view.

So, if I take those study results at face value, I see a very different association. Either their diet is deficient or they're suffering from some chronic medical condition, or both, and supplements merely palliate.