点击显示 收起
June 12, 2000 -- In any office, ask women about morning sickness. Every story will be different. "I never really threw up a lot, but I was so nauseous I wished I could have," says Lisa, mother of two. "I couldn't stand the smell of bacon." Angie felt sick every day of her pregnancy, every time she brushed her teeth. Durga carried a little brown bag with her on the New York subway system and used it just about every day. Yet Sylvia never felt sick, even once.
In a recent study, two Cornell University researchers examine the common phenomenon known as "morning sickness," which they christen with the acronym NVP (nausea and vomiting in pregnancy.)
"Morning sickness is not a sickness," says study author Samuel M. Flaxman, a graduate student in evolutionary biology at Cornell University in Ithaca, N.Y. "It's a normal part of many pregnancies and may in fact potentially benefit women and unborn children. And it's also not restricted to morning. It happens all day long, during waking hours."
His paper, co-authored by Paul W. Sherman, PhD, a professor of neurobiology and behavior at Cornell, reviews studies of nearly 80,000 pregnancies in 16 countries around the world. It appears in The Quarterly Review of Biology.
Not every pregnant woman experiences morning sickness. And among those who do, there is virtually no pattern. "That's why it's so intriguing to us," Flaxman says. "Why are there so many differences, so many variations in experiences?"
Morning sickness may serve to protect both the mother and the unborn child from potentially harmful things in foods, he says. "She expels foods that have nasty chemicals and microorganisms. She also learns to avoid these things in the future, at least in the short term." Pretty soon, the very smell, not to mention the taste, of certain foods makes her sick.
On the other hand, persistent, severe nausea and vomiting can be associated with serious medical conditions such as thyroid disease, infection, and a condition called hyperemesis gravidarum, which can lead to dehydration. Women should tell their physicians if they are unable to eat, if they are not gaining weight, or if they are feeling ill.
Nearly two-thirds of women experience morning sickness during the first trimester of pregnancy, Flaxman says. That makes sense in terms of fetal development, he tells WebMD. While the fetus is vulnerable to toxins throughout pregnancy, the first three months are the most critical. "It's a time of rapid development, growth, differentiation of tissues. When those processes are happening so rapidly, the fetus is more susceptible to insults."
Also, during pregnancy, a woman's immune system becomes weaker. Biologically, "this is necessary because the fetus is in some ways a foreign tissue," Flaxman says. "Half of its genome comes from the father. To prevent her from rejecting that tissue, she needs to be immunompromised. But that leaves her and her baby at increased risk of infection."
In fact, his review showed that women who have morning sickness tend to have more successful pregnancies than those who do not: "They were significantly less likely to miscarry. Stillbirths were also significantly less likely."
Researchers have debated the physiological mechanism that triggers morning sickness. Another recent study suggests that morning sickness is indeed a protective feature that creates "nutritional partitioning," Rachel Huxley, a public health researcher in England's Oxford University, tells WebMD. Her study is published in the journal Obstetrics and Gynecology.
Nutritional partitioning is a natural mechanism that ensures the fetus gets important nutrients from the mother's diet yet protects it from the mother's increased fat intake.
When a woman becomes pregnant, her levels of certain growth factors rise, encouraging her to take in more fat and boosting her metabolism of fat. "There's a very strong drive for fat intake, to provide for lactation," Huxley says. "But this puts the development of the early placenta at risk." Hormones released by the placenta appear to be key triggers of the nausea and vomiting during morning sickness, perhaps keeping the mother's food intake in check.
When the mother expels food, nutritional partitioning ensures that the placenta is not affected in an unhealthy way, Huxley tells WebMD. "The fetus receives priority treatment; it receives the nutrients it needs, so it develops very well. When the mother resumes her normal eating patterns, she again receives plenty of nutrients, as does the developing fetus."
"A mother who is obese or even normal weight will have a higher level of hormone [than a lower-weight woman], so she might feel worse than the woman who is lighter weight," Huxley adds.
But if a woman is underweight at conception, even if her fat levels increase during pregnancy, the fetus will be smaller, she says. "The body's drive to build fat reserves in the mother are very, very strong. It's much stronger drive than the placental drive, so the placenta suffers. ... It's not the full reason why some babies are born low birth weight, but it's one."
Hormones are likely one controlling factor for nausea and vomiting during pregnancy, Flaxman says. "Although reproductive hormones are necessary for the expression of NVP, they are not, by themselves, sufficient to trigger NVP symptoms. Certain sensory sensitivities -- both taste and smell -- may also be factors as the mother's body "trains itself to stay away from those things," he tells WebMD.
In his study, he found that women's strongest dislikes in the first trimester were meat, fish, poultry, and eggs. Across the board, the women were eating more fruits, sweets and dairy products, as well as grains and starches. In fact, in seven non-industrial societies in which anthropologists have observed no morning sickness symptoms, Flaxman says, "women were significantly more likely to have only plants rather than meat as a staple, especially corn or bland, starchy vegetables."
A meat and protein aversion in early pregnancy makes sense in evolutionary terms, he says. "Especially prior to the widespread use of refrigeration, meats were especially susceptible to colonization of microorganisms, so they were often sources of food-borne illnesses ... salmonella and E. coli." Another possibility is that certain culinary practices, such as frying, broiling and smoking, create harmful chemicals.
Morning sickness also protects the fetus against harmful chemicals -- even vitamins -- naturally found in plants, says Flaxman. "Vitamin A is a classic example; it's good for adults, good for eyesight, but in large quantities it can cause birth defects, can be carcinogenic. It can work much differently in a fetus because it's developing so rapidly."
Spices in food can have a similar effect, he adds. "Spices in food decrease the threat of food-borne disease because they have antimicrobial effects. But some spices in large quantities can be carcinogenic, toxic. They can have a quite different effect on the fetus than on adults."
He hastens to add, "Don't cut animal products from your diet based on these findings. Proteins are key sources of amino acids and lots of other nutrients that are critical to the success of a pregnancy." And today, the food supply is much safer than in years past. "There probably is not the threat in food that we've faced in evolutionary history."
Across generations, morning sickness appears to serve an important function: to naturally expel food for a good reason. Trying to alleviate the symptoms may interfere with this process. But it's a question of degree, he adds. "Some women have severe morning sickness; they're incapacitated by it. They can't keep food down. You obviously have to relieve symptoms at that point because you have to eat something. And comfort is important."
And women shouldn't think that morning sickness is necessary for a successful pregnancy, says Flaxman. "Most women, regardless of whether they have morning sickness, carry their pregnancy to term. You might not encounter that much nasty stuff in your diet. And even if you do, it's not like this is the body's only defense mechanism. We have enzymes that detoxify these things. And even though the immune system is compromised, it's still in place and operating."
Women with persistent nausea or vomiting, or who do not gain weight, should contact their physicians. They should also discuss with their doctors whether they should take prenatal vitamins.
To learn more, visit WebMD's Birth Stages Calculator, or the Weight Gain Estimator.
Vital Information: