The idea of becoming pregnant while already pregnant seems perplexing. Human reproduction is a complex process with built-in safeguards to ensure the healthy progression of a single pregnancy. While the body typically prevents new conception once one is underway, rare exceptions exist, challenging our understanding of reproductive biology.
The Body’s Protective Mechanisms
Once a pregnancy is established, the female body undergoes physiological changes to prevent further conceptions. A primary mechanism involves hormonal shifts. Pregnancy hormones, particularly progesterone, suppress the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. This halts ovulation by preventing the ovaries from maturing and releasing additional eggs.
Beyond ovulation, physical barriers also contribute. A thick mucus plug forms within the cervical canal, blocking sperm passage into the uterus and fallopian tubes. The uterine lining, or endometrium, also changes. Once an embryo implants, the endometrium becomes decidualized, making it unreceptive to subsequent embryo implantation. This altered environment makes it difficult for a new embryo to attach and thrive.
Understanding Superfetation
Despite the body’s robust protective measures, an extremely rare phenomenon known as superfetation can occur. Superfetation is the fertilization and implantation of an ovum in the uterus while another pregnancy is already in progress. This results in two fetuses of different gestational ages, conceived at separate times, often weeks apart.
Its occurrence in humans is exceptionally uncommon, with only 10 to 14 documented cases reported. Superfetation is observed more frequently in some animal species, such as rodents, hares, and fish.
For superfetation to happen, several improbable events must align. First, normal hormonal suppression preventing ovulation during pregnancy must fail. If an egg is released, it needs fertilization by sperm that bypasses the cervical mucus plug. Finally, the new embryo must successfully implant in a uterus already supporting another pregnancy, an environment typically unreceptive. While naturally rare, some cases link to assisted reproductive technologies, which can override natural controls.
Distinguishing Multiple Pregnancies
Superfetation is often confused with more common multiple pregnancies, but important distinctions exist. Fraternal, or dizygotic, twins result from the fertilization of two separate eggs by two different sperm during the same menstrual cycle. These twins develop independently, each with its own placenta and amniotic sac, and are no more genetically similar than other siblings.
Identical, or monozygotic, twins originate from a single fertilized egg that splits into two embryos early in development. These twins share nearly identical genetic information and are always the same biological sex. Unlike superfetation, both fraternal and identical twins are conceived at the same time. Superfecundation involves the fertilization of two or more ova released during the same ovulatory cycle by separate acts of intercourse, potentially from different partners. In superfecundation, offspring are the same gestational age, differentiating it from superfetation where conceptions occur at different times.