When to take a pregnancy test

Pregnancy tests detect hCG, a hormone that's only made after implantation. Here's the timeline:

1. Fertilization can happen on the day of ovulation or the day after.
2. The embryo travels to the uterus and implants 6 to 12 days after ovulation. In a study by Wilcox and colleagues (1999), most implanted on days 8 to 10.
3. After implantation, hCG rises quickly and reaches the threshold of home tests in urine within a few days.

That's why a test taken a few days after sex will be negative even if you did conceive.

From your last period: your expected period = first day of your last period + your cycle length. Ovulation happens about 14 days before your next period, because the second half of the cycle (the luteal phase) is fairly constant.

• Earliest test day: 10 days after ovulation, and only with a sensitive early-result test.
• Reliable test day: the first day of your missed period. The NHS and the US Office on Women's Health say most home tests are reliable from this day.
• Retest: if it's negative and your period still hasn't come a week later, test again.

From the day you had sex: if you don't know when you ovulated, the NHS suggests testing 21 days after sex.

For IVF, we work out the fertilization day from your transfer date and embryo age (day 3 or day 5). The beta-hCG blood test is usually about 14 days after that.

Your last period started on October 1 and your cycle is 30 days long.

If your cycle length varies, base the reliable test day on your longest cycle. For irregular cycles, the Office on Women's Health suggests testing 36 days after the start of your last period.

In IVF, your clinic will give you a beta-hCG blood test date, and that's the one to follow. After a day-5 embryo (blastocyst) transfer it's usually 9 to 10 days later, and after a day-3 transfer 11 to 12 days later.

Home tests can be especially misleading here. If you had an hCG trigger shot to release your eggs, the hormone can stay in your body for 10 to 14 days and cause a false positive. A negative result that's simply too early can cause needless worry. A blood test gives an exact value, and two results show how fast it's rising.

• First morning urine: in the early days it has the most hCG because it concentrates overnight. After a missed period, any time of day works.
• Don't drink a lot beforehand: diluted urine can make the line fainter.
• Read it on time: a faint line that appears after the reading window (an evaporation line) doesn't count.
• A faint line counts: if it shows up within the reading time, it usually means positive. Test again in two or three days.
• Negative but your period is late: wait a few days and test again. If it's still negative and your period hasn't come, see your doctor.

After a positive test, book a visit with your OB-GYN, family doctor or midwife. Start or keep taking 400 micrograms of folic acid a day if you aren't already. Our due date calculator works out when your baby is due, and the prenatal visit schedule shows which checkups and tests come when.

Get care right away if you have severe or one-sided belly pain, pain at the tip of your shoulder, dizziness or heavy bleeding with a positive test.