Doctors typically use a combination of ultrasound imaging, blood tests, and a pelvic exam to determine whether a miscarriage has occurred or is in progress. No single test gives a definitive answer in every case, and the process often requires repeat testing over several days to distinguish a miscarriage from a healthy but earlier-than-expected pregnancy.
Transvaginal Ultrasound
Ultrasound is the most important tool for diagnosing miscarriage in the first trimester. In nearly all cases, doctors use a transvaginal probe rather than a standard abdominal scan because it produces a much clearer picture this early in pregnancy. Abdominal ultrasound can reliably spot a gestational sac, but it detects an embryo only about 68% of the time and picks up a heartbeat in roughly 71% of cases where one exists. Higher BMI reduces accuracy further: for every 10-point increase in BMI, the chance of missing an embryo nearly triples. Transvaginal imaging avoids most of these limitations.
Doctors look at two key measurements. If an embryo is visible and measures 7 mm or more from crown to rump but has no heartbeat, that confirms a nonviable pregnancy. If no embryo is visible at all and the gestational sac measures 25 mm or more in mean diameter, that also confirms pregnancy loss. These thresholds, established by the Society of Radiologists in Ultrasound, are intentionally conservative. Earlier guidelines used smaller cutoffs (5 mm for crown-rump length, 16 mm for sac diameter), but the current numbers were adopted to virtually eliminate the risk of misdiagnosing a viable pregnancy.
Some findings raise concern without being conclusive on their own. A fetal heart rate below 100 beats per minute at 5 to 7 weeks of gestation and subchorionic hemorrhage (a blood collection near the gestational sac) are both associated with pregnancy loss but are not enough to make a definitive diagnosis.
What Happens When Results Are Unclear
Often, especially very early in the first trimester, the initial ultrasound falls short of those diagnostic thresholds. The sac may be too small to measure conclusively, or it may be too early to expect a visible heartbeat. When this happens, doctors classify the pregnancy as “of uncertain viability” and schedule a follow-up scan. The typical waiting period is 7 to 14 days. This gap gives a healthy pregnancy enough time to show clear growth, while a failing pregnancy will show no progress or further signs of loss.
This waiting period can be agonizing, but it exists for a critical reason: rushing to a diagnosis risks ending a pregnancy that might actually be viable. Doctors will not diagnose a miscarriage based on a single inconclusive scan.
Blood Tests for hCG Levels
Human chorionic gonadotropin, or hCG, is the hormone your body produces after a fertilized egg implants. Doctors use serial blood draws, typically 48 to 72 hours apart, to track how hCG levels change over time. The pattern matters more than any single number.
In a healthy early pregnancy, hCG levels roughly double every 2 to 3 days during the first four weeks, then slow to doubling about every 96 hours after six weeks. A rise of 53% or more over two days confirms a viable pregnancy in about 99% of cases. When hCG levels are declining, say from 120 mIU/mL down to 80 mIU/mL two days later, that indicates the pregnancy is no longer developing. Levels that rise but far too slowly (for example, 120 to 130 over two days) suggest a nonviable pregnancy where miscarriage may follow soon.
hCG testing is especially useful when an ultrasound is inconclusive or when the pregnancy is too early to visualize on imaging at all. It also plays a role in ruling out ectopic pregnancy, since certain hCG patterns combined with the absence of a gestational sac in the uterus can point to a pregnancy developing outside the uterus instead.
Pelvic Exam
A pelvic exam gives doctors physical information that imaging and blood work can’t fully capture. The main thing they check is whether the cervix has begun to dilate or thin. In a healthy early pregnancy, the cervix stays firm and closed. If it’s opening and you’re experiencing bleeding or cramping, that shifts the diagnosis from a “threatened” miscarriage (where loss is possible but hasn’t happened) to an “inevitable” miscarriage (where loss is underway and can’t be stopped).
Doctors also look for visible tissue at the cervical opening and assess how much bleeding is occurring. These findings, combined with ultrasound and blood results, help determine what type of miscarriage is happening and what steps come next.
Types of Miscarriage and How They’re Identified
The testing process looks slightly different depending on the type of loss:
- Missed miscarriage: The embryo has stopped developing or never formed, but the tissue remains in the uterus. There may be little or no bleeding, and many people have no idea anything is wrong until an ultrasound reveals no heartbeat. This is diagnosed entirely through imaging.
- Incomplete miscarriage: Some pregnancy tissue has passed, but some remains in the uterus. Ultrasound shows retained tissue, and bleeding and cramping are typically ongoing.
- Complete miscarriage: All pregnancy tissue has passed. Ultrasound shows an empty uterus, and hCG levels are declining toward zero.
Ruling Out Ectopic Pregnancy
When hCG levels confirm a pregnancy but ultrasound can’t locate a gestational sac in the uterus, doctors must determine whether the pregnancy is ectopic (growing in a fallopian tube or elsewhere outside the uterus). This is a medical emergency if the ectopic pregnancy ruptures, so it’s treated with urgency.
The combination of hCG trends and ultrasound findings is the primary screening approach. If hCG levels are above a certain threshold (typically 1,500 to 2,000 mIU/mL) and no intrauterine pregnancy is visible on transvaginal ultrasound, ectopic pregnancy becomes a serious concern. In some cases where tissue has been surgically removed, pathologists examine it under a microscope for specific structures called chorionic villi. If none are found, that raises the likelihood of an ectopic pregnancy and may prompt further investigation.
Genetic Testing After a Loss
After a miscarriage, doctors may recommend chromosomal analysis of the pregnancy tissue, particularly if you’ve experienced two or more losses. This testing can reveal whether a genetic abnormality caused the loss, which it does in roughly half of all first-trimester miscarriages. Knowing the cause can guide planning for future pregnancies and help determine whether additional testing of either parent is warranted.
If tissue is collected during a surgical procedure or passed at home and preserved, it can be sent to a lab for analysis. The two main methods are traditional karyotyping, which maps all the chromosomes, and chromosomal microarray, which detects smaller genetic changes that karyotyping might miss. Your doctor will advise on whether this testing makes sense based on your history and circumstances.