When a young woman reaches puberty, she starts to ovulate. This is when a mature egg or ovum is released from one of the ovaries. The ovaries are the two female reproductive organs found in the pelvis. If the egg is fertilized by a sperm as it travels down the fallopian tube, then pregnancy occurs. The fertilized egg attaches to the lining of the uterus. The placenta then develops. The placenta transfers nutrition and oxygen to the fetus from mother. If the egg does not become fertilized, the lining of the uterus (endometrium) is shed during menstruation. The average menstrual cycle lasts 28 days. The cycle starts with the first day of one period and ends with the first day of the next period. The average woman ovulates on day 14. At this time, some women have minor discomfort in their lower abdomen, spotting, or bleeding, while others do not have any symptoms at all. A woman is generally most likely to get pregnant (fertile) if she has sex a few days before, and during ovulation. What is menstruation?Menstruation is one part of a woman's cycle when the lining of the uterus (endometrium) is shed. This occurs throughout a woman's reproductive life. With each monthly cycle, the endometrium prepares itself to nourish a fetus. Increased levels of estrogen and progesterone help thicken its walls. If fertilization does not occur, the endometrium, along with blood and mucus from the vagina and cervix make up the menstrual flow that leaves the body through the vagina during the period. When does menstruation start?On average, a young woman in the U.S. has her first menstrual period at about age 12. This is generally 2 to 3 years after her breasts start to grow. This is also soon after she notices pubic and underarm hair. Stress, strenuous exercise, and diet can affect when a girl first has her period. The American College of Obstetricians and Gynecologists recommends that a young woman consult her healthcare provider if she has not started to menstruate by the age of 15, or if she has not begun to develop breast buds, pubic hair, or underarm hair by the age of 13. How long is a menstrual cycle?For menstruating women, an average menstrual cycle lasts 28 days. It starts with the first day of the last period and ends with the first day of the next menstrual period. However, the length of women's cycles varies, particularly for the first year or 2 after a young woman has her first period. Women may have cycles as short as 21 days, or as long as 45 days during the first few years. However, anything outside of this range may require medical attention.
If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. In humans, a pregnancy occurs when an ovum (egg) is fertilized by sperm. The fertilized ovum then implants itself in a woman’s uterus. For superfetation to happen, another completely different ovum needs to be fertilized and then implanted separately in the womb. For this to happen successfully, three very unlikely events need to take place:
The chances of these three unlikely events occurring simultaneously seem nearly impossible. This is why, of the few cases of potential superfetation reported in the medical literature, most have been in women undergoing fertility treatments. During a fertility treatment, known as in vitro fertilization, fertilized embryos are transferred into a woman’s uterus. Superfetation might happen if the woman also ovulates and the egg becomes fertilized by sperm a few weeks after the embryos are transferred into her uterus.
Can you get pregnant if you are already pregnant and would the result be twins? The term used to describe this situation is "superfetation." The babies are considered an unusual type of twinning. Superfetation is the formation of a fetus while another fetus is already present in the uterus. Essentially, it describes a situation where someone becomes pregnant when they are already pregnant. It is believed that this is a very rare event. Superfetation occurs when ova from two separate menstrual cycles are released, fertilized, and then implant in the uterus. Normally, once fertilization and implantation occur, physical and hormonal effects would make an additional conception impossible.
Hormones are the first barrier. They act to halt the process of ovulation and prevent the release of another egg from the ovaries. The uterine lining also changes after one embryo has implanted, making further implantation difficult. Another physical effect of pregnancy, the mucus plug, makes it difficult for sperm to breach the cervix during intercourse. Superfetation differs from the conception of typical twins, where multiple ova are released in a single cycle. This can happen naturally or be stimulated with fertility drugs. When more than one ovum is fertilized and implants in the uterus, the result is dizygotic (fraternal) twins, triplets, or other multiples. Although two fetuses develop simultaneously in superfetation, they differ in maturity, having been conceived days or even weeks apart. Superfetation is observed in animal reproduction, but it is exceedingly rare in humans. Only a few cases are documented in medical literature. Superfetation is suspected only when the twins are of different sizes and at different stages of development. It is typically noticed during a routine checkup on the ultrasound. However, it can be hard to distinguish whether this is a true case of superfetation or due to other factors.
Superfetation is different from superfecundation. This similar-sounding term describes twins conceived with sperm from two different sources. In superfecundation, conception of each fetus occurs on the same day or within a few days; the eggs are from the same menstrual cycle. In 2016, Australian Kate Hill became pregnant with two babies and had only had unprotected sex once. She had been receiving hormone treatments for polycystic ovary syndrome (PCOS), a condition that decreases ovulation. Doctors discovered that her twins were conceived 10 days apart and the fertilization was the result of just one ejaculation of sperm. What makes this case even more unusual is that it's generally accepted that sperm have a lifespan of around 5 days. Julia Grovenburg is an Arkansas woman who reportedly became pregnant with two babies due to superfetation in 2009. An ultrasound revealed that she was pregnant with two babies conceived about two and a half weeks apart. A case of superfetation in a triplet pregnancy was described in The Journal of Pediatrics in 2005. In this case, a 32-year-old woman became pregnant with the aid of fertility treatments. After two transferred embryos produced a twin pregnancy, a third fetus was discovered several months into the pregnancy and determined to be developing about three weeks behind the other two babies. If the two fetuses are at very different lengths of gestation, there is a risk for the younger one to be delivered prematurely. As there is a higher rate of twins being born at less than 37 weeks, this increases the risk for the younger twin.
Most of the verified cases have been with twins that are only 10 days to three weeks apart, and the reported cases were usually safely delivered. Verified cases of human superfetation are rare. While it may be an interesting phenomenon, it isn't one to worry about happening in your pregnancy.
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