Pregnancy is measured in several different ways depending on the stage: first by detecting it with a urine or blood test, then by dating it (figuring out how far along you are), and finally by tracking fetal growth through ultrasound and physical exams. Each method gives you a different piece of the picture, and they build on each other as pregnancy progresses.
Detecting Pregnancy With Home Tests
Home pregnancy tests work by detecting a hormone called hCG in your urine. Your body starts producing hCG shortly after a fertilized egg implants in the uterus, and levels rise rapidly in the first weeks. The key difference between home tests is how sensitive they are to low levels of this hormone.
Not all tests are equal. A study comparing over-the-counter brands found that First Response Early Result could detect hCG at concentrations as low as 6.3 mIU/mL, picking up more than 95% of pregnancies by the day of a missed period. Clearblue Easy Earliest Results needed 25 mIU/mL, catching about 80% of pregnancies at that point. Five other brands required 100 mIU/mL or more, which meant they detected only 16% or fewer pregnancies on the day of a missed period. If you’re testing early, the brand matters.
For the most reliable result, test with your first urine of the morning, when hCG is most concentrated. Testing too early, even with a sensitive kit, can produce a false negative simply because your body hasn’t produced enough hormone yet.
Blood Tests and hCG Levels
A quantitative blood test measures the exact amount of hCG in your bloodstream, which helps confirm pregnancy and gives a rough sense of how far along you are. Unlike a home test that gives a yes-or-no answer, a blood draw returns a number. Typical ranges by week of gestation look like this:
- 3 weeks: 5 to 72 mIU/mL
- 4 weeks: 10 to 708 mIU/mL
- 5 weeks: 217 to 8,245 mIU/mL
- 6 weeks: 152 to 32,177 mIU/mL
- 7 weeks: 4,059 to 153,767 mIU/mL
- 8 weeks: 31,366 to 149,094 mIU/mL
- 9 weeks: 59,109 to 135,901 mIU/mL
- 10 weeks: 44,186 to 170,409 mIU/mL
- 12 weeks: 27,107 to 201,165 mIU/mL
Notice how wide those ranges are. A single hCG number can’t pinpoint an exact week. What matters more in early pregnancy is the trend. In a healthy pregnancy, hCG levels typically double every 1.4 to 3.5 days, though the doubling time slows as levels climb higher. If your provider orders two blood draws 48 to 72 hours apart, they’re looking at whether the number is rising at the expected pace rather than focusing on the absolute value.
Estimating Your Due Date
The most common starting point for dating a pregnancy is the first day of your last menstrual period (LMP). By standard convention, the estimated due date is set at 280 days (40 weeks) from that date. This calculation assumes a 28-day cycle with ovulation on day 14, which doesn’t apply to everyone. If your cycles are irregular or you’re unsure of the date, this estimate can be off by a week or more.
You can do this math yourself: take the first day of your last period, add seven days, then count back three months. That gives you an approximate due date. Many pregnancy apps and online calculators automate this for you, but they’re all using the same 280-day formula.
First-Trimester Ultrasound Dating
The most accurate way to date a pregnancy is an ultrasound performed in the first trimester. During this scan, the technician measures the length of the embryo from the top of the head to the bottom of the torso, a measurement called crown-rump length. In these early weeks, embryos grow at a remarkably consistent rate regardless of genetics or the mother’s size, which is why the measurement is so precise.
First-trimester ultrasound dating is accurate to within roughly five to seven days. If the ultrasound date and the date calculated from your last period are close, your due date typically stays the same. If there’s a significant gap between the two, the ultrasound date usually takes priority because it’s more reliable.
Second and Third-Trimester Growth Measurements
Later in pregnancy, ultrasounds shift from dating to growth monitoring. Technicians take four standard measurements of the fetus to estimate size and weight:
- Biparietal diameter (BPD): the width of the baby’s head, measured from one side of the skull to the other
- Head circumference (HC): the distance around the baby’s head
- Abdominal circumference (AC): the distance around the baby’s belly
- Femur length (FL): the length of the thighbone
These four numbers are plugged into formulas that estimate the baby’s weight and compare it to expected ranges for that gestational age. This is how providers check whether a baby is growing on track, too small, or larger than average. Because babies vary more in size as pregnancy progresses, these later measurements aren’t as useful for pinpointing a due date as a first-trimester scan. They’re better suited for tracking growth patterns over time.
Fundal Height: A Low-Tech Check
Starting around week 24, your provider will likely measure fundal height at each prenatal visit. This is simply the distance in centimeters from your pubic bone to the top of your uterus, measured with a tape measure while you lie on your back.
The rule of thumb is straightforward: after 24 weeks, the measurement in centimeters should roughly match the number of weeks you are pregnant, give or take about 3 centimeters. So at 30 weeks, a fundal height of 27 to 33 centimeters would be considered normal. A measurement that falls outside that range doesn’t necessarily mean something is wrong, but it may prompt your provider to order an ultrasound for a closer look at the baby’s size or the amount of amniotic fluid.
How These Methods Work Together
No single measurement tells the whole story. Home and blood tests confirm that you’re pregnant. Your last period gives a preliminary timeline. A first-trimester ultrasound sharpens the due date. Later ultrasounds track whether the baby is growing as expected. And fundal height provides a quick, visit-to-visit check between scans.
Each method has its own window of usefulness. hCG levels matter most in the first few weeks, ultrasound dating is most precise before 14 weeks, and growth measurements become the focus in the second and third trimesters. Together, they give you and your provider a progressively clearer picture from the first positive test to delivery day.