Pregnancy is detectable when levels of the hormone Human Chorionic Gonadotrophin (HCG) reach significant levels in blood or urine. The hormone is secreted by the implanted embryo, and rises rapidly in the early stages of pregnancy. Globally there are approximately 211 million pregnancies per year and 137 million live births per year (WHO data). Rapid pregnancy tests are the primary method by which physicians and consumers establish that conception has taken place.

The menstrual cycle starts on the first day of the period and ends the day before the next period starts. For most women in the middle of reproductive years, their cycle usually lasts between 21 and 35 days. There are three phases in the cycle: the follicular phase, ovulation, and the luteal phase.

While the luteal phase (after ovulation) stays fairly constant at 14 days, the follicular phase can vary in length. Conception occurs when the egg released in ovulation is met with active sperm in the presence of high levels of the hormone estrogen. Approximately one week after conception the egg implants itself into the uterine wall and produces hCG to maintain the pregnancy -- hCG is the hormone detected by pregnancy tests.

Secretion of hCG starts at around the time of implantation of the fertilised egg. Concentrations of hCG then continue to rise during the early weeks of pregnancy reaching a peak between weeks 7-12. Low levels of hCG can be detected in urine. Based on published literature, these levels are expected to reach at least 50mIU/ml on the day of the missed period. Early testing is possible for many women because the hCG levels in their urine reach detectable levels before the day the period is due. By contrast, urine samples from healthy males and non-pregnant females generally contain less than 5mIU/ml hCG. The appearance and rapid rise in the level of hCG makes it an excellent marker for pregnancy. All pregnancy tests are based on the detection of hCG, either in serum (professional tests only) or in urine (both professional and home tests).

Early diagnosis of pregnancy aids the planning of early neonatal care, reducing the risks to mother and fetus, which allows for: Moderation/cessation of alcohol consumption and smoking, optimization of folic acid intake, avoidance of infections or contraindicated treatments, identification of suspected ectopic pregnancies, and fertility monitoring.