You asked

When to take a pregnancy test?

Pregnancy test timing can be complicated. Let's start at the beginning so you understand when to take a pregnancy test and how it works.
Pregnancy tests are designed to detect the hormone 'human chorionic gonadotropin' (hCG). This is known as the 'pregnancy hormone' and is generated by your body when you are pregnant. hCG is not generated in the body until implantation takes place. Since the timing of implantation - and even ovulation - varies not only from woman to woman, but month to month, taking even a sensitive test too early can give misleading results.
There are some tests sensitive enough to detect hCG levels prior to a missed pregnancy -  hCG levels double every couple days in early pregnancy - but if you are looking for accurate results you're better to try to wait until you have missed your period before you take a pregnancy test.  If you take the test too early, you may indeed be pregnant (or about to become pregnant), but the test will tell you that you aren't (a 'false negative').
Additionally, many early pregnancies end in miscarriage. Often you are not even aware that you were pregnant if this happens. Then you miscarry around the time your period was due. You simply think you were late and that your period is a bit heavy. Thus if you take the test too early, you run the risk that you will be burdened with the knowledge of a miscarriage you otherwise would not have known about.

All in all, it is better to just wait until you have missed your period to run a pregnancy test.
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More questions

The best timing for pregnancy testing is once you miss your period.
Ectopic pregnancies are fertilized eggs which implant themselves outside of the uterus. They must be terminated and can indicate future fertility issues.
Ovulation is the period during which you can conceive. Tracking ovulation can be tricky, but you can increase your chances of conceiving through math, prediction kits, or simple scientific observation.
Implantation bleeding is not a common experience, but some women do report small amounts of blood with or without cramping roughly a week or less before their anticipated period.
Pregnancy tests are only as good as the execution. Read all instructions to avoid getting inaccurate results on a home pregnancy test.
The best over the counter kits measure luteinizing hormone (LH).
There are many things that you can do to increase your chances of getting pregnant.
How STD’s can affect your pregnancy, your baby and how you should deal with them during pregnancy.
Taking care of your body before and during pregnancy
Ovulation occurs just about two weeks after the first day of your period.