You are not eating for two
The single most repeated piece of pregnancy advice is also the least accurate. ACOG puts the additional requirement at only a few hundred calories a day — roughly a substantial snack, not a second dinner. That figure is small enough that most people meet it without trying, and treating it as a licence to double up is one of the more common ways weight gain drifts outside the recommended range.
The requirement is also not evenly spread. In the first trimester it is close to nothing, which is convenient given how many people spend it unable to face food at all. It rises through the second and third, so the appetite most people notice around the midpoint is the body asking at roughly the right time.
What changes far more than the calorie count is what those calories should contain. Requirements for several specific nutrients rise sharply while overall energy needs barely move, which is why the useful question is what you are eating rather than how much.
Folic acid, and why the timing matters so much
Folic acid is a B vitamin that reduces the risk of neural tube defects — serious malformations of the developing brain and spine. ACOG recommends 600 micrograms daily during pregnancy, and notes this is difficult to reach from food alone, which is why a supplement is standard advice rather than a fallback.
The timing is the part people miss. The neural tube closes very early, so the recommendation is to take a prenatal vitamin containing at least 400 micrograms starting at least a month before conception where that is possible, and through the first twelve weeks. If a pregnancy was not planned, starting as soon as you know is still worth doing — the advice is to begin now rather than to conclude the window has closed.
Some people need considerably more than the standard dose, including those with a previous pregnancy affected by a neural tube defect, some forms of diabetes, and those taking certain anti-epileptic medications. That is a conversation with a provider rather than a decision to make from a label.
Iron, and why you may be told you are low
Iron requirements rise more steeply than almost anything else. ACOG puts the daily need at 27 mg during pregnancy against 18 mg outside it, because your body is producing a substantially larger volume of blood to supply the pregnancy.
Being told your iron is low is common and is not usually a sign that you have done anything wrong. Blood volume expands faster than red cell production, so a degree of dilution is expected and routine testing is designed to catch the cases that go beyond it. If you are prescribed a supplement, taking it with something containing vitamin C improves absorption, and taking it with tea or coffee reduces it.
Iron supplements very commonly cause constipation, which compounds a problem pregnancy already tends to produce. That is worth raising rather than tolerating — there are several formulations and alternate-day dosing is sometimes better absorbed as well as better tolerated.
Fish: genuinely worth eating, with specific exceptions
Fish is one of the few areas where the advice is often mangled into avoid entirely, and that is the wrong conclusion. ACOG recommends two to three servings a week during pregnancy, because the omega-3 fatty acids matter for fetal brain and eye development and are hard to obtain elsewhere.
The exceptions are specific rather than general, and they are about mercury, which accumulates in long-lived predatory species. ACOG advises avoiding bigeye tuna, king mackerel, marlin, orange roughy, shark, swordfish and tilefish, and limiting white albacore tuna to six ounces a week. Salmon, sardines, cod, tilapia, shrimp and light canned tuna are not on that list.
The practical version: eat fish regularly, and learn the seven names rather than avoiding the whole category out of an unspecific worry.
Listeria, and why the cheese advice exists
The reason soft cheeses, deli meats, pâté and unpasteurised products appear on every avoid list is a single bacterium. Pregnant people are roughly ten times more likely than the general population to become ill from listeria, and infection during pregnancy can cause miscarriage, stillbirth and serious newborn illness even when the parent is only mildly unwell.
Listeria is unusual among foodborne bacteria in that it grows at refrigerator temperatures, which is why the advice targets ready-to-eat chilled foods rather than raw ingredients. Heating to steaming hot kills it, so a deli meat that has been thoroughly cooked and a soft cheese baked into a dish are treated differently from the same items eaten cold.
Precisely which cheeses are cautioned varies between countries, because pasteurisation norms differ. This is one to check against guidance where you live rather than against a list from somewhere else — a rule that is correct in the United States is not automatically correct in France.
Vitamin D, and the supplements to be careful with
Beyond folic acid and iron, vitamin D is the one most commonly recommended outright. The UK advice is a daily supplement of 10 micrograms throughout pregnancy, on the basis that it is difficult to get enough from sunlight and diet — particularly through winter, and particularly for anyone with darker skin or who covers up outdoors. Guidance elsewhere is less uniform, and a standard prenatal vitamin usually contains some already.
The more important point about supplements is that more is not better, and a few are actively cautioned. High-dose vitamin A in the retinol form can cause birth defects, which is why liver and liver products such as pâté appear on avoid lists and why you should check that any general multivitamin you already take is not adding retinol on top of a prenatal one.
Herbal supplements and teas deserve the same scepticism as anything else. Being sold as natural says nothing about whether it has been studied in pregnancy, and for most of them it has not. If you take something regularly, mention it at your appointment rather than assuming it is too minor to raise — the point of asking is that you are not expected to know which ones matter.
When nothing stays down
For a substantial number of people the first trimester makes all of the above theoretical. Nausea and food aversions are common, they frequently target exactly the foods that were the plan, and the correct response is not to force the ideal diet.
In that window, eating anything tolerable beats eating nothing, and the calorie requirement is close to unchanged anyway. Small and frequent tends to work better than three meals, cold food is often easier than hot because it smells less, and keeping something plain within reach for the moment you wake up helps some people. A prenatal vitamin taken at night is sometimes kept down when the same one at breakfast is not.
There is a line beyond which this stops being ordinary morning sickness. If you cannot keep fluids down, are losing weight, or are passing very little urine, that is worth contacting someone about the same day rather than waiting — hyperemesis is treatable and the treatments work better before dehydration is established.