A pregnant woman should eat for two.
No additional calories for the first trimester, 340 kilocalories (per day) the second trimester, and 452 kilocalories (per day) for the third trimester.
That said, recommendations vary based on maternal BMI and pre-pregnancy nutritional status. It is also interesting to note:
Hytten (1980) estimated the energy cost of pregnancy to be 85,000 kcal.
That is the equivalent of 425 Larabars! Amazing. Good thing the preggo has ten months to eat that much extra food.
Energy requirements are greatest between 10 and 30 weeks of gestation, when relatively large quantities of maternal fat normally are deposited. Substantial fetal demands (56 kcal/kg per day) are offset in the last quarter of pregnancy by the near cessation of maternal fat storage (Sparks et al., 1980).
In an interesting parallel, premenstrual women have increased caloric needs that outstrip the increased caloric needs of their premenstrual peers. Check out this excerpt from a study on basal metabolic rates throughout a woman's menstrual cycle. In the premenstrual period, scientists found:
RMR was 0.99 +/- 0.16 kcal/kg/h. The energy expenditure while sitting was 1.06 times RMR, while walking it was 2.81 times RMR, and while performing treadmill exercise it was 3.47 times RMR.
This means that your increased interest in ALL THE FOOD is far more (metabolically) appropriate in the days preceding your period than it is during pregnancy. Of course, what you need to eat and what you want to eat are often different things and this is not a prescription about how you must eat. Use these data to make choices about how you eat and I urge you to not read into this data-oriented post any sort of shame about how you should eat while pregnant. If you read this and decide to eat more than you need or differently than what is simply nutrient-oriented, that is your prerogative. When you do indulge, enjoy.
So next time someone shames you for how you eat or look, tell them to step off - and share a bit of hard science. After all, science is the most effective antidote to misogyny.