An abnormal pregnancy is a medical situation where a gestation deviates from the expected normal progression, location, or developmental timeline. This deviation means the pregnancy cannot proceed to a healthy, live birth or presents a significant health risk to the pregnant individual. The term encompasses a range of complex conditions involving the fertilized egg, the developing embryo, or the placental tissues.
Understanding the Scope of Abnormal Pregnancy
Medical professionals classify pregnancies as abnormal when there is a failure of correct implantation, proper development, or continuation of viability. Viability refers to the potential for a fetus to survive outside the uterus, which becomes a clinical consideration later in gestation, generally after 22 to 24 weeks. A non-viable pregnancy, particularly in the first trimester, indicates that a live birth is impossible due to developmental or location issues.
Two common terms for pregnancy loss are differentiated by timing: a miscarriage, or spontaneous abortion, is the loss of a pregnancy before 20 weeks of gestation. A stillbirth is the term used for fetal death occurring at or after the 20th week of pregnancy. Abnormalities may be caused by issues with the location of the implantation, the genetic structure of the embryo, or the development of the supporting placental tissue.
In the earliest stages, a pregnancy may be considered non-viable if the human chorionic gonadotropin (hCG) hormone levels do not rise as expected, even before an embryo is visible on ultrasound. This often points to a fundamental problem with the fertilized egg itself, such as a chromosomal error, or to a failure of the uterine environment to support the developing conceptus.
Distinct Categories of Abnormal Pregnancy Conditions
Ectopic Pregnancy
An ectopic pregnancy is defined as the implantation of a fertilized egg outside of the main cavity of the uterus. This condition is life-threatening because the location cannot support the growth of an embryo. Approximately 97% of ectopic pregnancies occur within one of the fallopian tubes, which is why they are frequently called tubal pregnancies.
As the gestation attempts to grow, it can cause the fallopian tube to rupture, leading to severe internal bleeding. Immediate medical or surgical intervention is necessary. Other, less common sites for ectopic implantation include the ovary, the cervix, or the abdominal cavity.
Molar Pregnancy (Gestational Trophoblastic Disease)
A molar pregnancy, also known as a hydatidiform mole, is an unusual growth that forms in the uterus at the beginning of a gestation. It is characterized by an abnormal fertilization event that results in the overgrowth of placental tissue, called trophoblasts. This tissue looks like a mass of fluid-filled cysts resembling a cluster of grapes and develops instead of a normal fetus and placenta.
There are two primary types of molar pregnancies: complete and partial. In a complete mole, the genetic material is entirely paternal, and there is no developing embryo or normal placental tissue. A partial mole results from the fertilization of one egg by two sperm, leading to an abnormal genetic structure that may show some early, non-viable fetal parts alongside the abnormal placental growth. Following removal, close monitoring of hCG levels is required because this abnormal tissue can persist or develop into a malignant form called choriocarcinoma.
Chromosomal and Genetic Abnormalities
Chromosomal abnormalities involve errors in the number or structure of the 46 chromosomes that carry the genetic instructions for development. These genetic errors often occur spontaneously during the formation of the egg or sperm cells, or during the earliest stages of cell division after fertilization. Such defects disrupt normal fetal growth and viability, frequently resulting in early pregnancy loss.
One common type of numerical error is a trisomy, where an individual has three copies of a particular chromosome instead of the usual two. Examples include Trisomy 21 (Down syndrome) and Trisomy 18 (Edward syndrome), which is associated with severe developmental challenges and is often life-limiting. The severity of the challenges and the likelihood of viability depend directly on which chromosome is affected and the extent of the genetic error.
Contributing Factors and Increased Risk
A number of factors can increase the likelihood of an abnormal pregnancy, though many cases occur without an identifiable cause. Advanced maternal age is a well-established risk factor, particularly for chromosomal abnormalities. The quality of an individual’s eggs may decrease over time, making errors in cell division, such as those that cause trisomies, more probable for those over the age of 35.
Pre-existing maternal health conditions can affect the ability to carry a pregnancy to term. Poorly controlled chronic conditions like diabetes, hypertension, and certain thyroid disorders can contribute to complications, including stillbirth and miscarriage. A history of previous pregnancy loss also increases the risk for a subsequent abnormal gestation.
Structural or environmental factors often play a direct role. Conditions that cause scarring or damage to the fallopian tubes, such as prior pelvic inflammatory disease (PID) or a history of pelvic surgery, can prevent the fertilized egg from traveling to the uterus (leading to ectopic pregnancy). Exposure to teratogens—substances that can cause birth defects, including certain medications, alcohol, or infectious agents—can interfere with normal development.
Diagnosis and Clinical Management
The diagnostic process for a suspected abnormal pregnancy relies on a combination of specific imaging and laboratory tests. Transvaginal ultrasound (TVUS) is the primary imaging tool used in early pregnancy to confirm the location of the gestation and assess its viability. TVUS determines if the pregnancy is located inside the uterus and if fetal heart activity is present.
Simultaneously, serial blood tests measure the level of human chorionic gonadotropin (hCG), the hormone produced by the placenta. In a normal, developing pregnancy, hCG levels are expected to rise significantly over a 48-hour period. A level that plateaus or declines, or a level above the “discriminatory zone” (typically 1,500 to 2,000 mIU/mL) without a visible intrauterine sac on TVUS, raises suspicion for an ectopic or other non-viable pregnancy.
Management pathways are individualized and depend on the specific diagnosis. For an ectopic pregnancy, treatment options include medical management using a methotrexate injection, which stops the growth of the abnormal cells, or surgical intervention to remove the pregnancy. Methotrexate is a non-surgical option that resolves most early ectopic pregnancies.
Abnormal intrauterine gestations, such as miscarriages or molar pregnancies, typically require the removal of the abnormal tissue from the uterus. This is commonly achieved through a procedure called dilation and curettage (D&C). In some stable cases of miscarriage, expectant management may be appropriate, allowing the body to pass the tissue naturally. Genetic abnormalities identified later in gestation are managed through specialized counseling and care plans tailored to the specific condition and the needs of the family.