Pregnancy Confirmation
Early Pregnancy Diagnosis: When to Test and When an Ultrasound Is Needed
If you suspect you may be pregnant or have a positive home pregnancy test, it is natural to have questions about what comes next. Early pregnancy diagnosis focuses on confirming whether a pregnancy exists, where it is located, and whether it is developing normally.
How Is Pregnancy Confirmed?
Pregnancy is initially confirmed by detecting the hormone human chorionic gonadotropin (β-hCG) in the urine or blood.
When Does a Pregnancy Test Become Positive?
With sensitive pregnancy tests, β-hCG can be detected as early as 8–9 days after ovulation, which may be several days before a missed period. However, test sensitivity varies depending on the brand and detection threshold. Additionally, blood β-hCG tests are more sensitive than urine tests.
A positive urine or blood pregnancy test confirms pregnancy, but it does not provide information about whether the pregnancy is viable and the location of the pregnancy (inside or outside the uterus).
When Is an Ultrasound Used to Confirm Pregnancy?
Therefore, an ultrasound is a useful and necessary adjunct to a pregnancy diagnosis. An ultrasound is used to confirm where the pregnancy is located, establish the gestational age of the embryo or the fetus, determine the number of embryos, and to assess whether the pregnancy is developing normally. It is also used to evaluate other pelvic structures, such as the presence of uterine fibroids or ovarian cysts.
In very early pregnancies, the preferred method of ultrasound is transvaginal (TVS), as it provides better visualization of the uterus and the developing embryo which is still quite small.
What Can Be Seen on Early Pregnancy Ultrasound?
Early pregnancy ultrasound can demonstrate a series of findings which go in chronological order. At ~4.5 weeks, an intrauterine gestational sac can generally be seen. This is a pregnancy sac inside the uterus, and it is the first visible sign of pregnancy. At ~5.5 weeks, the yolk sac becomes visible. This looks like a little ring within the gestational sac. Further on, at approximately 6-6.5 weeks, the developing embryo can generally be visualized.
It is important to distinguish a true gestational sac from a pseudosac, which is a fluid collection within the womb cavity that may occur in ectopic pregnancy. If this finding is seen without other confirming features—especially in women with pain or bleeding—further evaluation is required.
Fetal Heartbeat in Early Pregnancy
Fetal cardiac activity may be detected when the embryo measures as small as 2 mm. The fetal heart activity should be visible once the embryo measures 7 mm in a viable pregnancy.
If on ultrasound the fetal cardiac activity is not yet seen, further evaluation may be needed, including repeating the ultrasound in 1 or 2 weeks, or blood testing, depending on gestational age, size of the embryo and clinical symptoms.
Is Early Pregnancy Ultrasound Safe?
Yes. Diagnostic ultrasound is considered safe in pregnancy. Studies have demonstrated no causal relationship between ultrasound and adverse pregnancy outcomes. Despite that, it is recommended to perform an ultrasound only when medically indicated, using the ALARA (as low as reasonably achievable) principle to minimize exposure.
What are the reasons to perform an early pregnancy or first trimester ultrasound?
A first-trimester ultrasound may be recommended to confirm an intrauterine pregnancy, estimate gestational age and determine the number of embryos, confirm cardiac activity, and evaluate for early fetal anomalies and nuchal translucency (when appropriate). It is also used in the case a pregnant woman presents with bleeding to evaluate for ectopic pregnancy or miscarriages. Furthermore, an early pregnancy ultrasound may be needed to assist with procedures such as embryo transfer or CVS.
Early pregnancy assessment can provide clarity and reassurance during the first weeks and is therefore recommended to schedule an ultrasound once you have a positive pregnancy test.
References
1. Cunningham FG, Leveno KJ, Dashe JS, Hoffman BL, Spong CY, Casey BM. Williams obstetrics. 26th ed. New York: McGraw-Hill Medical; 2022.
2. International Society of Ultrasound in Obstetrics and Gynecology, Bilardo CM, Chaoui R, Hyett JA, Kagan KO, Karim JN, Papageorghiou AT, Poon LC, Salomon LJ, Syngelaki A, Nicolaides KH. ISUOG Practice Guidelines (updated): performance of 11–14-week ultrasound scan. Ultrasound Obstet Gynecol. 2023;61(1):127-43. doi: 10.1002/uog.26106.
Maternal–Fetal Medicine Specialist in Bangkok
Dr. Narisra Srikureja Firer (Dr. Niti)
Dr. Niti is a Bangkok-based OBGYN with subspecialty training in Maternal–Fetal Medicine (MFM). She provides comprehensive care for women across all stages of life, including pregnancy care, deliveries, and general gynecological conditions.
She dedicates her time to private practice at Ruamjairak Hospital and Mission Hospital, and serves as a Maternal–Fetal Medicine Consultant at a large government tertiary care center.
To read Dr. Niti’s full bio, click here.
