Doctors have a few diagnostic tests to determine if you are pregnant. The examination may include a medical history, pregnancy tests (i.e. urine or blood), a physical/ pelvic exam and diagnostic imaging (e.g. ultrasound). While I’ve heard of some doctors leading with physical exam, they more frequently go the less invasive route of a pregnancy test first. Imaging can also be a useful tool in the early stages of pregnancy, but that is typically used after a positive pregnancy test.
How does pregnancy testing work?
It’s a good idea to discuss how pregnancy tests work. There are two main types of pregnancy tests: urine pregnancy test and blood test. Both tests detect HCG levels in the body. HCG, or human chorionic gonadotropin, is a hormone secreted during pregnancy which stimulates continued production of progesterone. After the sperm meets the egg and travels down the fallopian tube, the egg fertilized egg attaches to the uterine wall. During this process, the body begins to produce HCG in order to alert the body to continue to produce hormones necessary to sustain pregnancy. HCG levels rise rapidly as a healthy pregnancy progresses, doubling every 48-72 hours until they reach a plateau between 8 and 11 weeks of pregnancy.
A home test react to the level of HCG present in urine. The sensitivity of each test varies depending on the brand. All tests have a “threshold” in which they will show a positive result based on the level of HCG in the urine. HCG hormone is measured in milli-international units per milliliter (mIU/mL). A woman is judged to be “not pregnant” with an HCG level of less than 5 mlU/ML and pregnant with a level of 25mIU/ML or higher. Most urine pregnancy tests will display a positive result with an HCG level of 25mIU/ML or greater. Some brands, like First Response, claim to be a more sensitive test than other brands so it’s possible that their threshold is lower. However, most tests can detect the presence of HCG in urine about 12-14 days after conception, which is right around the time of a missed period. In my experience, most doctors will have you take this type of test first if you’re coming in under “normal” or routine circumstances.
Blood tests have a higher sensitivity and can detect pregnancy earlier than most home pregnancy tests. They are usually performed in the doctor’s office because they have to collect a blood sample to test. A doctor may immediately order a blood pregnancy test if you have a history of miscarriage/pregnancy loss, you have certain preexisting health conditions, or you’re working with a fertility specialist (e.g. IVF).
There are two types of blood tests: Qualitative and Quantitative. Qualitative HCG blood tests are basically as accurate as a urine test because they check to see if the pregnancy hormone is present or not. Quantitative (or the beta HCG test ) blood tests measure the exact amount of HCG in the blood allowing it to find even the tiniest amounts of HCG. This makes the test extremely accurate and effective at detecting a potential pregnancy very early on. These are the tests most frequently used by doctors.
How soon can a doctor tell if you're pregnant?
Now that we have some background knowledge, we can better answer the question. We’ve already determined that a quantitative blood test would be the best way to get the earliest positive pregnancy result. Every woman’s HCG levels rise at their own rate so it all just depends on how long it would take for the pregnancy hormone levels to reach or surpass 25mIU/ML in the blood. The American Pregnancy Association has a chart that shows the possible range for HCG levels by the week. Based on the chart, a doctor could, in theory, detect a pregnancy as early as three weeks (HCG range 5-50mlU/ML) since your last period or approximately one week after ovulation.
Although this is hopeful, I would use caution when interpreting this information. If you look at the chart, the beginning of the ranges for each week up until week 6 is a number lower than 25 mlU/ML. This means that it is possible for a woman who is pregnant to still get a negative result up until 5 weeks of pregnancy due to slow moving HCG level rise. Also, if a healthcare professional saw levels between 5 and 50 on an initial test, they would likely proceed with caution before officially diagnosing a pregnancy. While 25 mlU/ML and above is indicative of pregnancy, a doctor will likely want to see that those levels are continuing to increase over the next few days as the pregnancy progresses.
It is possible for a doctor to detect a potential pregnancy as early as one week (or 7 days) after ovulation. However, it is very likely that a health care provider will do ongoing testing and monitoring of early signs of pregnancy (e.g. morning sickness, other first trimester symptoms) to confirm a healthy and viable pregnancy. Under ideal circumstances, you would likely get a doctor’s pregnancy confirmation a few days earlier than a standard home pregnancy used on the first day of your expected period.