A biochemical pregnancy (BP) is a very early pregnancy loss that occurs shortly after the fertilized egg successfully implants in the uterine wall. The term “biochemical” means the pregnancy is only detectable by the presence of human chorionic gonadotropin (hCG) in a blood or urine test. This loss happens before a gestational sac or embryo can be seen on an ultrasound, typically before the fifth week of gestation. Biochemical pregnancies are common, accounting for an estimated 50 to 75 percent of all miscarriages, though many go unnoticed and are mistaken for a slightly delayed menstrual cycle.
Chromosomal Abnormalities in Early Embryos
The most frequent reason for a biochemical pregnancy is an inherent defect within the developing embryo, primarily chromosomal abnormalities. An estimated 50 to 70 percent of early miscarriages are attributed to an incorrect number of chromosomes, known as aneuploidy. These genetic errors render the embryo non-viable, leading the body to naturally halt development shortly after implantation.
This genetic misstep usually occurs during meiosis, the cell division process that creates the egg and sperm cells. Errors result in an egg or sperm that contains too many or too few chromosomes. When these abnormal cells combine, the resulting embryo carries a genetic imbalance, such as an extra chromosome (trisomy) or a missing chromosome (monosomy).
An embryo with a severe aneuploidy is recognized by the body as incapable of forming a healthy fetus. Implantation may start, generating enough hCG for a positive test, but the lack of developmental potential quickly causes the pregnancy to fail. This mechanism prevents the progression of pregnancies that cannot survive beyond the earliest stages. Such defects are often random occurrences in the specific egg or sperm cell and do not reflect a chronic problem for the parents.
Impaired Uterine Environment and Hormonal Support
Beyond the quality of the embryo, a successful early pregnancy depends on a supportive environment within the uterus and sufficient hormonal signaling. The inner lining of the uterus, called the endometrium, must achieve a state of “receptivity,” a brief window of time where it is prepared to accept the implanting embryo. This readiness is regulated by a balance of hormones, primarily estrogen and progesterone.
Progesterone plays a central role by transforming the uterine lining into a thick, highly vascularized tissue capable of sustaining the conceptus. After ovulation, the corpus luteum produces this progesterone. If there is a functional issue with the corpus luteum, insufficient progesterone production can prevent the endometrium from fully maturing or stabilizing the early pregnancy after implantation.
Physical issues within the uterine cavity can also interfere with the embryo’s ability to embed correctly, even if the embryo is genetically normal. Structural abnormalities like submucosal fibroids or endometrial polyps can disrupt the implantation site, making the environment less hospitable. Additionally, some underlying immune responses may cause the mother’s body to identify the embryo as foreign and reject it.
Maternal Health Conditions and Lifestyle Contributors
Certain pre-existing health conditions and lifestyle choices increase the risk by influencing the embryo’s quality or the uterine environment’s receptivity. Advanced maternal age is a factor, as egg quality declines over time, increasing the likelihood of chromosomal errors (aneuploidy). For women aged 40 and older, over half of their embryos may carry a chromosomal abnormality.
Systemic diseases that affect hormonal balance or immune function are also implicated in early loss. Untreated thyroid disorders, particularly hypothyroidism, can disrupt the hormonal milieu necessary for maintaining the uterine lining. Poorly managed diabetes or Polycystic Ovary Syndrome (PCOS) can also lead to hormonal imbalances that negatively impact the development of a receptive endometrium.
Lifestyle factors contribute to the overall risk by affecting gamete quality and the uterine environment. Excessive caffeine intake, smoking, and significant weight variations (underweight or obese) have been linked to an increased risk of early pregnancy loss. Addressing underlying health conditions and making positive lifestyle changes can help optimize the reproductive environment for future successful implantation.