What Is an Anembryonic Pregnancy and Why Does It Occur?

An anembryonic pregnancy, also known colloquially as a blighted ovum, is one of the most common causes of first-trimester miscarriage. It happens when the gestational sac forms but the embryo never develops. Although patients may experience symptoms similar to a normal pregnancy, the ultrasound shows an empty sac. In this guide, we explain in detail what it is, why it happens, and how to detect it early—so you have all the information you need before consulting your specialist at Fertility Madrid.

What Is an Anembryonic Pregnancy?

Anembryonic pregnancy: characterized by the presence of a gestational sac with no embryo inside.
Blighted ovum: colloquial term describing the empty sac.
Non-viable pregnancy: a broader category including any embryo that fails to be detected after several weeks.

Difference from Other Miscarriage Types

Biological Mechanism: Why Does a “Blighted Ovum” Occur?

Failure of cell division
- After fertilization, the zygote must divide; a chromosomal error halts this process.
Chromosomal abnormalities
- 50–70 % of cases are linked to unviable karyotypes (trisomies, monosomies).
Implantation issues
- Alterations in the endometrium or embryo-uterine signaling prevent embryonic development.

When these disruptions happen very early, the sac continues to grow but the embryo does not, resulting in a blighted ovum.

3. Requirements to Be an Egg Donor

3.1 Medical Requirements

Age: Most clinics set an age range, typically between 18 and 30 years, to maximize egg quality and quantity.
General Health: The donor must be in good health with no history of hereditary diseases or conditions that could affect the process.
Genetic and Hormonal Evaluations: Tests are performed to rule out genetic risks and evaluate ovarian reserve.

3.2 Legal and Ethical Requirements

Informed Consent: The donor must sign an informed consent document outlining the process, risks, and ethical implications of the donation.
Confidentiality: The donor’s identity is kept confidential and protected, ensuring privacy rights are upheld.
Ethical Aspects: It is essential that the process is conducted under strict ethical guidelines, ensuring the donation is voluntary and non-profit.

3. Associated Risk Factors

Advanced maternal age
- From age 35 onward, the incidence of chromosomal abnormalities rises.
History of previous miscarriages
- Two or more prior losses slightly increase the risk of another blighted ovum.
Environmental and lifestyle factors
- Smoking, toxin exposure, oxidative stress.
Immune issues or infections
- Though less common, certain immune dysregulations can affect early embryonic development.

Prevalence and Likelihood

According to various studies, 1 in 5 pregnancies ends up anembryonic before eight weeks—often before the patient even realizes she was pregnant.

Overall probability: approximately 10–20 % of clinically recognized pregnancies.
Age-related risk:
- Under 30 years: ~ 12 %
- 35–40 years: ~ 20–25 %
- Over 40 years: > 30 %

Early Detection: Ultrasound and Symptoms

5.1 Most Common Symptoms

Light bleeding or spotting
Abdominal cramping
Rapid decline in β-hCG levels
Note: In many cases, the only sign is absence of a fetal heartbeat.

5.2 Diagnostic Protocol

Transvaginal ultrasound
- From week 6, the gestational sac should reveal an embryo with cardiac activity. An empty sac at this stage indicates a blighted ovum.
Hormonal monitoring
- β-hCG levels that fail to double every 48 hours.
Confirmation
- Repeat ultrasound at weeks 7–8 to rule out delayed development.