Primary infertility is the inability to ever bear a live child. Secondary infertility is the inability to bear a live child after having done so at least once before.
Secondary infertility means can’t have second child (or third, or fourth …).
The WHO (World Health Organization) maintains several definitions for infertility. There are unfortunately multiple qualifications to the definitions that can make them less than straightforward.
The WHO clinical (as opposed to demographic or epidemiological) definition of infertility comes from a 2009 glossary document it put together. It states that infertility is:
“a disease of the reproductive system defined by the failure to achieve a clinical pregnancy after 12 months or more of regular unprotected sexual intercourse.”
The WHO draws its specific definitions for primary and secondary infertility from a paper by Mascarenhas et al. in PLOS Medicine:
“Primary infertility is defined as the absence of a live birth for women who desire a child and have been in a union for at least five years, during which they have not used any contraceptives.”
“Secondary infertility is defined as the absence of a live birth for women who desire a child and have been in a union for at least five years since their last live birth, during which they did not use any contraceptives. … Women in a fertile union have successfully had at least one live birth in the past five years and … have been in a union for at least five years following their first birth.”
Note that miscarriages and stillbirths don’t count as live births, thus couples who get pregnant, but don’t deliver live babies would still be considered infertile.
I’m not exactly sure why the WHO maintains definitions of infertility that specify 1 year of trying to get pregnant and then other definitions that specify 5 years of trying. It’s likely true that different organizations have even more definitions that aren’t discussed here.
Also note that it gets even more confusing if you are talking about infertility related to a person vs. related to a couple. Usually one person in the couple is responsible for the lack of a baby, but the couple as a whole could be labelled as infertile. Suffice it to say that knowing the Bottom Line is probably sufficient. Primary infertility means there is no first baby.
The WHO website of definitions for words related to infertility
Wikipedia’s excellent page which uses the WHO definitions
The WHO glossary paper
The Mascarenhas et al. paper in PLOS Medicine