When am I pregnant?

You’ve either seen it in movies or had the honour yourself: a test strip you’ve had the pleasure of peeing on, reveals the happy news – Congratulations … you’re pregnant! Sometimes only discernible by squinting the eyes. Nonetheless, a signal of joy for those who wish to conceive. But what is the underlying science behind being pregnant? When exactly is a woman in the state of being pregnant? How can test strips detect that? And why is it good practice to urinate on a strip more frequently rather than not?

The definition

A woman is defined as pregnant when: after fertilisation of the egg (when sperm and egg meet), the zygote (fertilised egg) has settled in the lining of the uterus and develops into an embryo and subsequently into a foetus. A pregnancy is, therefore, the time frame in which a fertilised egg matures into an embryo or foetus.

Fertilisation of the ovum takes place in the upper third of the fallopian tube after the ovum has been released from the ovary into the fallopian tube (ovulation). The zygote then migrates through the fallopian tube into the uterus, where it then nests into the wall. Before the zygote nests, the two sets of chromosomes merge.

After the successful implantation into the endometrium, further cell division occurs and the early placenta (syncytiotrophoblast) develops. This part of the placenta produces the human chorionic gonadotropin or hCG. This hormone is important in two ways. It signals to the body that no oocytes should mature in the coming cycles and it can be detected via urine.

Thus, pregnancy tests which measure the hCG value allow the early determination of a pregnancy even before an embryo can be detected with an ultrasound. However, depending on the product, pregnancy tests have certain detection limits, sensitivity, and accuracy. The hCG value also gradually rises at first. Thus it’s advisable to test more than once in order to exclude false positive results.