Can a person become pregnant when a pregnancy is already underway? The short answer is yes, it is biologically possible, though this occurrence is profoundly rare in humans. This phenomenon, where a second conception successfully takes place days or weeks after the first, is known as superfetation. It results in two fetuses of different gestational ages developing simultaneously in the womb.
Biological Mechanisms That Prevent Second Conception
The reproductive system relies on three primary biological safeguards to prevent a new pregnancy once one is established. The first is hormonal suppression of ovulation. High levels of hormones like progesterone and human chorionic gonadotropin (hCG) signal the pituitary gland to halt the maturation and release of additional eggs from the ovaries. This hormonal environment switches off the monthly reproductive cycle, preventing a second fertilization event.
A second crucial defense is the physical blockage of the cervix. Once conception occurs, the cervix develops a thick, protective mucus plug that physically prevents sperm from traveling into the uterus and fallopian tubes. This barrier ensures that sperm cannot reach an egg for fertilization.
The third mechanism involves the uterine environment itself. The lining of the uterus, or endometrium, must be prepared to accept and nourish a newly fertilized egg, a process called implantation. Once an embryo has implanted, hormonal changes transform the endometrium to support the existing pregnancy, making it unreceptive to the implantation of a second embryo. Superfetation requires the failure of one or more of these standard mechanisms, allowing a second conception to take root.
Understanding Superfetation
Superfetation is defined as the successful fertilization and implantation of a second ovum while a viable pregnancy is already established. This means the two fetuses begin development at distinctly different times. For this to happen, the hormonal suppression that normally stops ovulation must fail, causing an egg to be released after the first pregnancy has begun. This second egg must then be fertilized by sperm that bypasses the cervical mucus plug.
Finally, the second fertilized egg must find a receptive spot in the already occupied uterine lining to implant and begin development. This chain of unlikely events makes documented cases of superfetation in humans extremely uncommon, with only a few confirmed instances reported. Superfetation is notably more common in some animal species, such as fish and hares. In humans, most reported cases have involved women who underwent assisted reproductive technologies, suggesting that fertility treatments may sometimes inadvertently override the body’s natural suppressive mechanisms.
Clarifying Similar Dual Gestation Concepts
Superfetation is often confused with other dual gestation scenarios, but it is unique because the second conception occurs during a subsequent reproductive cycle. One distinct concept is superfecundation, which involves the fertilization of two separate eggs released during the same menstrual cycle. These eggs are fertilized by separate acts of intercourse, potentially resulting in twins conceived days apart.
A rare variation, known as heteropaternal superfecundation, occurs when the two eggs are fertilized by sperm from two different biological fathers. In both cases of superfecundation, the pregnancies begin in the same cycle, unlike the weeks-apart timing of superfetation.
Another scenario that can cause confusion is Vanishing Twin Syndrome. This occurs when one of two embryos, conceived at the same time, stops developing early in the pregnancy and is reabsorbed by the body. Initial ultrasound scans might show twins of different sizes, but this is due to one twin ceasing growth, not a delayed second conception. Superfetation involves two different gestational ages, meaning the fetuses are at two different stages of development corresponding to two different conception dates.
Detection and Fetal Outcomes
The most significant indication of superfetation is the observation of two fetuses with a substantial difference in size and developmental stage during a routine ultrasound scan. Doctors use ultrasound to measure the fetuses, and a major discrepancy in growth rates, known as growth discordance, suggests two distinct estimated conception dates. This confirms that the embryos did not begin developing simultaneously.
The primary concern with a superfetation pregnancy is the risk of premature birth for the younger fetus. Since the fetuses have different gestational ages, the older one will reach full term maturity weeks before the younger one. Medical professionals face the challenge of determining the optimal delivery time, balancing the necessity for the older fetus to complete its development against the risk of the younger fetus being born significantly underdeveloped. Premature birth can lead to complications for the younger baby, including low birth weight and difficulties with lung development, requiring specialized care.