The beginning of pregnancy may be detected in a number of ways, including various pregnancy tests which detect hormones generated by the newly-formed placenta. Clinical blood and urine tests can detect pregnancy as early as 6-8 days after date of conception. Home pregnancy tests are personal urine tests, which normally can't detect a pregnancy until at least 12-15 days after conception. Both clinical and home tests can only detect the state of pregnancy, and cannot detect the actual date of conception.

In practice, for the purpose of giving a date for a conception (i.e. an "age" for an embryo), doctors typically date the pregnancy by "menstrual date," based on the first day of a woman's last menstrual period, as the woman reports it. Unless a woman's recent sexual activity has been limited, the exact date of conception or implantation are unknown. And absent any symptoms of morning sickness, the only visible

sign of a pregnancy is often an interruption of her normal monthly menstruation cycle, (i.e. a "late period"). Hence, the "menstrual date" is simply a common educated estimate for the age of a fetus, which is an average of two weeks later than conception, (the margin of error considers 0 to 30 days after last menstruation, hence a 14 day average). The term "conception date" may sometimes be used when that date is more certain, though even medical professionals can be imprecise with their use of the two distinct terms. An unknown date for conception means that in practice the distinction between embryo and fetus is a clinical one only, and not used as to refer to stages of development of a particular pregnancy.

There are likewise finer distinctions between the concepts of fertilization (conception) and the actual state of pregnancy. In a normal pregnancy, the fertilization of the egg usually will have occurred in the Fallopian tubes or in the uterus. (In women with fertility problems, an egg may become fertilized yet fail to become implanted in the uterus.) If the pregnancy is the result of in-vitro fertilization the fertilization will have occurred in a Petri dish, after which "pregnancy" begins when one or more zygotes implants after being transferred by a physician in the woman's uterus.

In the context of political debates regarding a proper definition of life, the terminology of pregnancy can be confusing. Because precise assessment of a pregnancy as being at the "embryo" or "fetus" stage is usually undeterminable, the terms (though more clinically precise) are less commonly used than terms like "baby" or "child." The medically and politically neutral term which remains is simply "pregnancy," though this can be problematic as it only refers indirectly to the embryo or fetus. In the context of personal treatment, bedside manner generally dictates that doctors make sparse use of clinical language like "fetus" and "embryo," and instead simply refer to the developing child as a "baby."

Childbirth is the process in which the baby is born. It is considered by many to be the beginning of a person's life, where age is defined relative to this event in most cultures.

A woman is considered to be in labour when she begins experiencing regular painful uterine contractions, accompanied by changes of her cervix — primarily effacement and dilation. While childbirth is widely experienced as painful, some women do report painless labours.