The idea that a woman needs to eat for two while pregnant is "a commonly misunderstood myth".
This is according to Jacqui Clinton, Tommy's health campaigns manager.
According to Ms Clinton, an unborn baby takes everything that he or she needs from the mother's body.
"It's not until the third trimester, from when you are six months pregnant, that you may need a little extra food, but only up to 200 extra calories a day, which isn't much."
However, women should be careful that these extra calories come from a health source, rather than empty foods.
The comments follow the publication of research from The University of Texas Health Science Center San Antonio, which revealed that foetal brain development in a nonhuman primate model can be impaired if a mother eats less during early pregnancy.
Among the problems seen in the brains of foetuses whose mothers had been fed a reduced diet during the first half of pregnancy were decreased formation of cell-to-cell connections, cell division and amounts of growth factors.