Many adults do not have biological children. Sometimes this is a matter of choice and sometimes a consequence of infertility.
Infertility is the failure to conceive for at least one year of unprotected sex, or the inability to carry a live pregnancy to birth. Infertile people are described as involuntarily childless. Some will subsequently conceive naturally, while others are not able to without reproductive assistance. Some will never conceive. About 7% of women remain childless because of female and male infertility, while 16–20% of couples are affected by infertility at some point in their lives.
The conditions that cause male and female infertility are biological and social. Biological female infertility is mainly caused by gynaecological problems, such as tubal defects and endometriosis. Biological male infertility has been linked to impaired sperm production. For both sexes other biological factors associated with infertility include hormonal imbalances, exposure to sexually transmitted diseases and environmental pollutants, and long-term use of certain contraceptives. Lifestyle factors that contribute to infertility include smoking, rapid weight loss and obesity.
Gender is not a significant factor in infertility. About 30% of infertility cases are caused by female factors alone and 30% by male factors alone. Another 30% are caused by combined female and male factors, and 10% of infertility cases cannot be explained.
Age and infertility
Age is a critical factor in infertility, and is particularly relevant for women. Their peak reproductive age is between 19 and 25, and fertility drops significantly from their mid-30s. In the early 21st century most women start families when their fertility is declining. This is because of social factors and lifestyle choices such as women entering and staying in the workforce, and using the contraceptive pill. In 2015 the median age of women giving birth was 30. The many women who wish to start a family in their mid- to late 30s are likely to have more difficulty conceiving than younger women, and some will remain childless or only have children through assisted reproduction, adoption or foster care.
Voluntary and contingent childlessness
Individuals and couples who are voluntarily childless or childfree may have children at a later date or decide not to have children at all. In the past voluntary childlessness was criticised as selfish, unfeeling and unpatriotic. It is more socially acceptable in the 21st century.
Some people are childless because of particular circumstances, rather than outright choice. ‘Contingent childlessness’ may result from being busy with other life activities such as career, education, financial commitments, partnership break-up, re-partnering and other family changes, or being in a same-sex relationship. In New Zealand women who live in cities and those with higher degrees are most likely to fall into this category.
New Zealand’s fertility rate is higher than most other OECD (Organisation for Economic Co-operation and Development) countries. In 2012 it was 2.1 per woman, making it the third equal-highest rate with Turkey after Israel and Mexico. The OECD average that year was 1.7.
Voluntary and contingent childlessness is more common than in the past, and accounts for New Zealand’s declining fertility rate, which has generally been at or just below replacement level (2.1 births per woman) since the early 1980s. This form of childlessness has increased from 1% of women born in 1936 to almost 10% of women born in 1965. Researchers estimate that 25% of women born in the mid-1970s may not have children – most through choice.