This is, quite simply, not true. Well, not entirely true, anyway.
Mamas, you need to be eating salt during your pregnancy. But please eat salt in natural ways; the need for sodium does not give you free rein to eat all of the fast food and processed packaged junk (most of which is completely loaded with salt) that you want. Rather, most caregivers recommend that you salt your food to taste, preferably using the highest-quality salt you can find. If you think your soup or your egg or your whatever needs a dash or two of salt at the table, then add some, by all means!
Here's a simplified explanation. Yes, salt does contribute to fluid retention. However, this is necessary during pregnancy. Most caregivers recognize that mild fluid retention in the feet and ankles is normal during pregnancy, even somewhat desirable, since it is a sign of your body having enough extra fluid. (Note that while swelling in the feet and ankles is normal, swelling in the arms and face is not, and should be brought to the attention of your caregiver immediately.) This extra bodily fluid is essential for your increased blood volume; while pregnant, you have about 40% more blood in your system, and limiting your sodium intake restricts this blood volume expansion. Cutting back on salt will not lower your risk for preeclampsia; on the contrary, it seems increase your risk, according to some studies. Additionally, you could be hampering your placenta's growth and potentially hurting your baby.
One of the best explanations I've seen regarding salt during pregnancy comes from the Brewer Diet. But every pregnancy book I own (and I own quite a few) that mentions salt specifically is in favor of including salt in your diet to taste.
From Pregnancy, Childbirth, and the Newborn by Penny Simkin:
From Eating Expectantly by Bridget Swinney:"Experts know that gradual, moderate water retention in pregnancy is not only normal, but the extra fluid is necessary for an adequate volume of blood and amniotic fluid. During pregnancy, consuming an adequate amount of salt helps maintain your fluid balance. Feel free to salt your food to taste."
From The Complete Book of Pregnancy & Childbirth by Sheila Kitzinger:"Sodium needs increase during pregnancy because of the extra fluid your body retains to cushion your baby... Some swelling is a normal part of pregnancy; cutting your sodium below what's recommended won't help and may hurt."
From Heart & Hands by Elizabeth Davis:"It used to be thought that salt was dangerous in pregnancy and a cause of preeclampsia, but when a group of expectant mothers were given no-salt diets, they had more preeclampsia than a control group who had as much salt as they wished."
From The Pregnancy Book by Dr. Sears (and also in the section "Satisfy With Salt" on AskDrSears.com):"Contrary to popular opinion, salt is a necessary nutrient and should be used according to taste."
From YOU: Having a Baby by Michael F Roizen:"Unless advised by your health-care provider, you should not restrict your salt intake while pregnant. Salt causes your body to retain fluid, of which you need more during pregnancy... Salt your food to taste."
And lest you think that I only own weird, non-mainstream pregnancy books, it's worth noting that even What to Expect When You're Expecting by Heidi Murkoff says salt in moderation is good. Even their website says "don't blame salt for those puffy feet." Here's what the book says:"Women may get cravings for salt because sodium is needed to balance their extra fluid volume during pregnancy."
Your Pregnancy, Week by Week by Glade Curtis isn't fully on board with the idea of salt to taste during pregnancy, but even they grudgingly admit that some is important."It's believed that some increase in bodily fluids in pregnancy is necessary and normal, and a moderate amount of sodium is needed to maintain adequate fluid levels. In fact, sodium deprivation can be harmful to the fetus."
So there you have it."You do need some [salt] every day to help deal with your increased blood volume."
Were you concerned about salt during your pregnancy?