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ScienceWeek
MEDICAL BIOLOGY: ON ADVANCED MATERNAL AGE IN HUMANS
The following points are made by Linda J. Heffner (New Engl. J. Med. 2004 351:1927):
1) The past decade has seen a remarkable shift in the demographics of childbearing in the US. The number of first births per 1000 women 35 to 39 years of age increased by 36 percent between 1991 and 2001, and the rate among women 40 to 44 years of age leaped by a remarkable 70 percent. In 2002, 263 births were reported in women between 50 and 54 years of age. Most media attention paid to older mothers has been favorable, inspiring a female business-school student to declare on a nationally televised segment of CBS's 60 Minutes (entitled "The Biological Clock"), "I plan to be super fit, super in shape when I'm 40, 50. And if I'm physically able to do it, then I will have a child at 55."
2) How realistic is it for a young woman today to expect to delay her childbearing into the fifth or sixth decade of her life while she pursues her career? The effect of maternal age on the outcome of pregnancy may be best assessed by examining five specific factors that can negatively affect the desired outcome of a pregnancy -- namely, a healthy mother and a healthy baby: declining fertility, miscarriage, chromosomal abnormalities, hypertensive complications, and stillbirth. Maternal death, the risk of which also increases with age, is fortunately so rare that it does not factor into this discussion. When examined according to the decade of life, the data on these reproductive outcomes speak for themselves.
3) Fertility is the rate of childbearing in a population. Fertility rates in populations that do not practice contraception best approximate the ability of women to conceive. The author uses results calculated on the basis of 10 different populations living between the 17th and the 20th centuries that did not use contraceptives. Fertility remains relatively stable through 30 years of age, at more than 400 pregnancies per 1000 exposed women per year, and then begins to decrease substantially. By 45 years of age, the fertility rate is only 100 pregnancies per 1000 exposed women.
4) Miscarriage is defined as spontaneous pregnancy loss before the 20th week of gestation. Karyotyping of the products of conception after miscarriage indicates that approximately two-thirds are chromosomally abnormal. At 20 years of age, the rate of miscarriage is approximately 10 percent. It increases to a high of more than 90 percent among women 45 years of age or older. This high miscarriage rate contributes significantly to decreasing fertility among older women. The effect of advancing maternal age on the risk of chromosomal abnormalities is well known.[3] Advanced paternal age, which is frequently associated with advanced maternal age, increases the risk of autosomal dominant diseases, such as achondroplasia and Marfan's syndrome, that appear to result from new genetic mutations. Deterioration in the quality of the ova with advancing maternal age is thought to be responsible for both the decline in fertility and the increasing risk of chromosomal abnormalities; recent experience with in vitro fertilization using donor eggs from younger women supports this hypothesis.[1,2,4,5
References:
1. Menken J, Trussell J, Larsen U. Age and fertility. Science 1986;233:1389-1394. [Erratum, Science 986;234:413.]
2. Anderson A-M, Wohlfahrt J, Christens P, Olsen J, Melbye M. Maternal age and fetal loss: population based register linkage study. BMJ 2000;320:1708-1712
3. Hook EB, Cross PK, Schreinemachers DM. Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA 1983;249:2034-2038
4. Salihu HM, Shumpert MN, Slay M, Kirby RS, Alexander GR. Childbearing beyond maternal age 50 and fetal outcomes in the United States. Obstet Gynecol 2003;102:1006-1014
5. Paulson RJ, Boostanfar R, Saadat P, et al. Pregnancy in the sixth decade of life: obstetrics outcomes in women of advanced reproductive age. JAMA 2002;288:2320-2323
New Engl. J. Med. http://www.nejm.org
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Related Material:
HUMAN EVOLUTION: THE GRANDMOTHER EFFECT AND HUMAN LONGEVITY
The following points are made by Kristen Hawkes (Nature 2004 428:128):
1) Also some people think postmenopausal women make little difference in the story of human populations, Lahdenper and colleagues et al(1) have unearthed firm evidence in support of the "grandmother hypothesis", according to which a grandmother has a decidedly beneficial effect on the reproductive success of her children and the survival of her grandchildren.
2) The question of human longevity has deeper evolutionary importance than many think. It is often assumed that the steady increase in life expectancy over the past century and a half(2) has resulted in a larger proportion of older people than ever before. But until the past few decades, increases in life expectancy reflected reductions in infant and juvenile mortality, and made little difference to the fraction of women past child-bearing age. It is levels of fertility, not life expectancy (mortality), that shift the proportion of elders in a population. Even when life expectancy is well below 40 years, most girls who survive childhood live past their child-bearing years. In both historical and hunter-gatherer populations, a third or more of women are usually beyond the age of 45.
3) This large proportion of older people has fundamental implications for all human social organizations. Its unusual character is highlighted by comparisons with other primates. For example, among our closest living relatives, chimpanzees, female fertility declines at about the same age as in people, from a peak before age 30 to virtually zero at age 45 (3). But chimpanzee survival rates fall along with fertility, so that in the wild less than 3% of the adults are over 45(4).
4) We might assume that the large fraction of elders in human populations reflects a characteristically human social safety net. But natural selection generally favors the flow of help from older to younger kin, so we should be skeptical that a species-wide pattern of care for older people explains human longevity(5). Developments in evolutionary life-history theory suggest that instead of help for older members of the population, it is help from postmenopausal grandmothers that accounts for the age structures of human societies.
References (abridged):
1. Lahdenper , M., Lummaa, V., Helle, S., Tremblay, M. & Russell, A. F. Nature 428, 178 181 (2004)
2. Oeppen, J. & Vaupel, J. W. Science 296, 1029 1031 (2002)
3. Nishida, T. et al. Am. J. Primatol. 59, 99 121 (2003)
4. Hill, K. et al. J. Hum. Evol. 39, 1 14 (2001)
5. Kaplan, H. K. Pop. Dev. Rev. 20, 753 791 (1994)
Nature http://www.nature.com/nature
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Related Material:
HUMAN LONGEVITY AND FAMILY SIZE
Notes by ScienceWeek:
What is called the "disposable soma theory" in the biology of ageing states that longevity of the organism requires investments in somatic maintenance (i.e., maintenance of the body) that reduce the resources available for reproduction. A possible corollary of this is that avoidance or prevention of reproduction will tend to increase longevity, and there are experiments with the fruit fly Drosophila melanogaster that indicate this trade-off exists.
The following points are made by R.G.J. Westendorp and T.B.L. Kirkwood (Nature 1998 396:743):
1) The authors report a study of the interrelationship between longevity and reproductive success in humans using historical data from the British aristocracy. The records used in the study involve the years 740 to 1875, and data concerning 19,830 male and 13,667 female aristocrats.
2) The authors report that the number of progeny was small when women died at an early age, increased with the age of death, reaching a plateau through the 6th, 7th, and 8th decades of life, but decreased again in women who died at an age of 80 years or over. Age at first childbirth was lowest for women who died early and highest for women who died at the oldest ages. When account as taken only of women who had reached menopause, who were aged 60 years and over, female longevity was negatively correlated with number of progeny and positively correlated with age at first childbirth.
3) The authors suggest the findings show that human life histories involve a trade-off between longevity and reproduction.
4) Commenting on this paper, in the same issue of the journal, Daniel E.L. Promislow (Nature 1998 396:719) states: "Although childless women benefit from a decreased incidence of heart disease and cervical cancer later in life, they have an increased risk of breast cancer and respiratory disease. Clearly, many factors influence lifespan."
Nature http://www.nature.com/nature
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