Of everything in fertility nutrition, trans fats have the least ambiguous evidence behind them, and they are also the easiest thing to spot once you know the three words to look for.
This page is mostly about how to read an ingredients list, which turns out to be the practical skill underneath most fat advice.
This is one of the few places on this site where a systematic review lands on a clear direction rather than a maybe.
Trans fats harm fertility.
A systematic review of the research supports this.
In the female TFAs intakes are associated with an increase in the risk of ovulatory infertility... high TFA intake (more than 1% of energy consumption) constitute a risk factor for infertility in both sexes.
Source: PubMed
In the US, in 2015. And they are still in food, because the ban has thresholds and a product under the threshold can round its label down to zero.
Which is why the ingredients list, not the nutrition panel, is where the answer is.
Trans fats were banned in the US in 2015 but are still found in everyday foods, and the average American likely still consumes them.
A health authority states this in its published guidance.
In 2015, FDA released its final determination that Partially Hydrogenated Oils (PHOs) are not Generally Recognized as Safe (GRAS)... trans fat will not be completely gone from foods because it occurs naturally in small amounts in meat and dairy products, and is present at very low levels in other edible oils.
Source: US Food and Drug Administration
"Hydrogenated oil," "partially hydrogenated oil," and "vegetable shortening" on an ingredients list are all trans fats and should be avoided.
A health authority states this in its published guidance.
Industrially produced trans fat can be found in margarine, vegetable shortening... Industrially produced trans fats are formed in an industrial process that adds hydrogen to vegetable oil, converting the liquid into a solid and resulting in partially hydrogenated oil (PHO).
Source: World Health Organization
Monounsaturated fat is the least controversial answer, and it comes from foods most people already like.
Monounsaturated fats are liquid at room temperature, almost always plant-derived, and found in olive oil, peanut oil, canola oil, avocados, nuts, and seeds.
A health authority states this in its published guidance.
Monounsaturated fats are liquid at room temperature but start to harden when chilled... found in plant foods, such as nuts, avocados, and vegetable oils... Foods and oils with higher amounts of monounsaturated fats include: Nuts, Avocado, Canola oil, Olive oil, Safflower oil, Sunflower oil, Peanut oil and butter, Sesam…
Source: NIH National Library of Medicine
This one does not have a clean answer, and pretending otherwise would be dishonest. What is established is arithmetic rather than a fertility effect, and it is below.
Full-fat dairy contains more saturated fat and calories than low-fat versions, so other meals may need adjusting to avoid weight gain.
A health authority states this in its published guidance.
It has been speculated that although full-fat dairy contains more calories and fat than reduced-fat dairy, full-fat versions may taste better and be more filling.
Carrageenan turns up in plant milks and is worth knowing about if your digestion has been unhappy, though the effect below is a subset of people rather than everyone.
Carrageenan can cause gastrointestinal inflammation in some people.
A systematic review of the research supports this.
carrageenans...may have adverse effects on gastrointestinal health, including aggravating intestinal inflammation...pre-existing conditions may predispose individuals to varied outcomes
Source: PubMed