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Special Article - Confinements resulting in multiple births
In the United States of America, 2.7% of all confinements resulted in a multiple birth in 1996 (U.S. Census Bureau, 1999, p.80)1. The proportion of confinements ending in a multiple birth in New Zealand was 1.6% in 1998 (Statistics New Zealand, 2000, p.70)2, slightly higher than in Australia (1.5% in 1998).
REASONS FOR THE INCREASE IN CONFINEMENTS RESULTING IN MULTIPLE BIRTHS
The two related factors widely regarded as the primary catalysts behind the increased number of confinements producing multiple births are older age at child bearing and the increased use of assisted conception techniques (AIHW 2000, p. 15)3 (NCHS 1999, p. 5)4.
Provided that current trends towards delayed parenthood and the increased availability of fertility treatments continue, it is likely that the occurrence of multiple births will increase further, at least in the short-term (NCHS, 1999, p. 6)4.
Age of mothers
Multiple births are more common among older mothers due to physiological reasons (NCHS, 1999, p. 5)4. With the median age of all mothers approaching 30 years, the increased age at childbearing has obvious implications for the incidence of confinements resulting in multiple births. The increase in confinements resulting in multiple births among older mothers (aged over 30 years) is disproportionate to the increase in the total number of confinements for the same age group. Between 1980 and 2000 the number of confinements resulting in multiple births to mothers aged 30 years and over increased two-fold (by 1,590 confinements) compared to a 4% (60 confinements) increase for mothers aged under 30 years. While the total confinements for mothers aged 30 years and over increased by 115% (64,000), total confinements for mothers aged under 30 years decreased by 25% (41,700) between 1980 and 2000.
The increasing tendency for women to have children at older ages reflects a general shift in attitudes towards family formation, reproduction and female participation in the labour force. The changing longevity and nature of partnerships may also be a factor.
CONFINEMENTS RESULTING IN MULTIPLE BIRTHS
The increased popularity and success of assisted conception (techniques such as in-vitro fertilisation (IVF), intracytoplasmic sperm injection (ICSI) and gamete intrafallopian transfer (GIFT) is believed to be a major factor in the increase in confinements resulting in multiple births. Between 1992 and 1999 there was a 105% increase in the number of viable pregnancies resulting from assisted conception. There were 3,900 births (including live births and still births) from assisted conceptions in Australia in 1998, an increase of 9% (360) from the previous year (AIHW 2001, p.26)3. In the same year, 690 (20%) of the 3,400 pregnancies of at least 20 weeks gestation (including pregnancies resulting in either a live birth or a still birth) from assisted conceptions resulted in multiple births; a figure far higher than the 1.5% of all pregnancies that resulted in multiple births in 1998. The occurrence of multiple births does not seem to differ greatly between IVF and ICSI pregnancies, although GIFT pregnancies result in a higher incidence of multiple births. In 1998 multiple births occurred in 28% of GIFT pregnancies, 20% of IVF pregnancies and 19% of ICSI pregnancies.
For further information on multiple births and births in general see Births, Australia, 2000 (Cat. no. 3301.0) which was released on 31 October 2001.
1 U.S. Census Bureau (1999), Statistical Abstract of the United States: 1999.
2 Statistics New Zealand (2000), Demographic Trends 1999.
3 Hurst T., Lancaster P. 2001, Assisted Conception, Australia and New Zealand 1998 and 1999. AIHW National Perinatal Statistics Unit and
4 National Centre for Health Statistics, National Vital Statistics Reports, Vol. 47, No. 24, September 14, 1999, National Vital Statistics
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