One thing that women don’t usually have to worry about when getting pregnant is getting pregnant. It is said to be one of the reasons that some couples enjoy doing it so much during those nine months - she’s already pregnant, so birth control is no longer a concern and pregnancy is no longer a worry.
A pregnant body is a sophisticated machine with the hormones and physical changes designed not only to support the growing fetus but also to prevent another baby being conceived and over stretching the resources of the mother. Nature has designed things so that generally one mom has one baby and that one child will have all of the support and nutrients all to itself. Although twins, triplets and very occasionally more do occur naturally, they are the exception.
However, just as with everything in life nothing is 100% guaranteed. Superfetation is the phenomenon where an already pregnant woman becomes pregnant again, and it is so rare that only around ten confirmed cases have ever been recorded in humans.
Never heard of it? You’re not alone; most people haven’t. So here is everything you could ever want to know about superfetation.
15It’s Not Unusual If You’re A Hare
Hares, badgers, minks, and even, some say cats, are all thought to be capable of superfetation. Since ancient times hares and their reproductive systems have been studied in an attempt to work out if becoming pregnant was a real phenomenon or whether there was another explanation for two or more offspring being born, slightly apart or together at different stages of fetal development.
With hares, the system for ovulation is different from that of humans. We have cycles of hormones that stimulate the egg release, but hares release eggs in response to sexual stimulation. Therefore, every time they have sex they release eggs and can carry fetuses of different gestations at the same time. This increases the number of offspring they can have in the limited period they are fertile.
We, however, have a much longer fertility period and do not need to cram in as many babies as possible. Indeed, one child at a time is optimal, and our systems are designed to prevent any other eggs and potential fetuses competing for scant resources.
14It’s Different Than Superfecundation
Superfetation is not the same thing as superfecundation either. What the heck is superfecundation do I hear you ask? Are you just making these words up now? Well, no, I’m not making them up, and superfecundation really is a thing - and it is the last extra long multi syllable term beginning with super that I am going to use, I promise.
Superfecundation is when a woman ovulates, and instead of releasing just one egg, she releases two. That same woman then has sex with two different men, either at the same time or separately, and one egg is fertilized by a sperm from man one, and the other egg is fertilized by a sperm from man two. Both eggs implant and grow, and the woman has fraternal twins. Two babies, eggs fertilized and implanted at the same time, different fathers, Superfecundation.
13Why It’s Not Just Twins
Twins are produced in two separate ways. First of all, there are fraternal twins where a woman releases two eggs at once, in one cycle and both eggs are implanted. They have separate sacs and placentas and are as alike, or not, as any other random siblings.
Identical twins are produced when one egg is fertilized by one sperm, and at some point in the early stages the tiny bundle of cells divides, and instead of remaining one bundle which continues to develop into one baby, it divides to far and splits completely in half making two separate bundles of cells. Both bundles are genetically identical, and both bundles go on to form a baby - identical twins.
In superfetation, a woman produces an egg that is fertilized by a sperm. It starts dividing, and implants into the uterus and the woman is pregnant. At some stage, after this, the woman goes through another hormonal cycle and releases another egg which is also fertilized by a sperm and implants. You now have two embryos from two entirely different events, both at different stages of fetal development.
12You Need To Ovulate While Pregnant
The first hurdle to getting pregnant when you are already expecting is that for a pregnancy to occur you have to have a newly released egg. To have a newly released egg, your have to have ovulated and although a new egg is released every month in a healthy fertile woman ovulation is suppressed once a woman is pregnant.
This happens in two stages. First, the corpus luteum, which is the tissue from which the egg is shed, hangs around for a couple of days pumping out hormones that prevent further ovulation. If the fertilized egg implants into the lining of the uterus, the corpus luteum sticks around a bit longer until the placenta is established, after which the placenta takes over.
For this reason, it should be impossible to ovulate again days or weeks after an egg has been fertilized.
11Ovulation Should Only Be Every 28 Days
The second hurdle to the entire process is timing. The average woman's body works on a 28-day cycle that goes like this.
The pituitary gland in your brain produces follicle stimulating hormone (FSH) which stimulates your ovaries to produce mature eggs. Your ovaries are full of fluid filled cavities each of which holds an egg. Once FSH is increased the number of follicles develop and pump out estrogen. One follicle will become the dominant one, and the egg matures while the estrogen thickens the lining of the womb ready to receive an egg.
All the rising estrogen causes a fast rise in the level of luteinising hormone, and this causes the dominant follicle to burst, releasing the egg and sending it off down the fallopian tube, and it dies after about a day if unfertilized.
Once the egg is released the follicle produces progesterone, further thickening the lining but if the lining isn’t needed it starts to break up, and you have a period.
Therefore it should be impossible to release a second egg a week or two after the first.
10Only Super Sperms Need To Apply
Of course, to get pregnant at all, you have to have your egg meet a guys sperm and have one of those sperm flex their tail a little more than the others and burrow into the egg. While this is what they are designed to do they do require specific conditions to do it well which is why at a certain point in your menstrual cycle the lubrication in your lady garden changes so that it optimal for the boys to swim and stay alive for a few days in the hope of snagging an egg.
With an existing pregnancy already there, the environment in your womb should not be conductive to hosting and supporting sperm, but there is still another problem.
When you are pregnant, your body keeps the growing baby safe from infection by sealing the entrance to the uterus with a thick plug of mucus. Not only should this keep germs out it should also prevent anything else getting in, such as sperm.
9The Body Shouldn’t Accept The Second Fetus
Even if you get past these two hurdles, there are still more to leap over before you can get pregnant again while pregnant.
There are particular environmental factors that must be just right before the fertilized egg can become implanted into the lining of a woman's uterus. An ideal lining will be at least 7mm thick on day of ovulation but less than 12mm on the day of ovulation. Estrogen and progesterone levels have to be at exactly the right level and even then a fertilized egg does not always take.
Once an embryo takes root and the placenta has developed, it is already getting pretty crowded in your uterus. If you have somehow ovulated again while pregnant AND have done so at an odd time in your cycle AND produced another egg AND that egg has been fertilized by a super sperm that has got past all of the hurdles, that embryo then has to find room to implant.
8It May Be A Delay
Some scientific theories have been put forward to give a different solution to the pregnant while pregnant puzzle. The idea has been put forward that it is not the result of two separate ovulation events that have taken place and implanted in the womb at the same time. Instead, so the theory goes, two eggs from the same cycle are fertilized but for some reason only one implants and begins development while the second hangs around for a while in a kind of strange suspended animation. Then, for some reason as yet undetermined the second fertilized egg ‘wakes up’ again, implants and then, hey presto, you have two embryos at different stages of development.
While some mammals are known to evolved to do this to ensure plenty of offspring. The exact mechanism by which it works is not understood and so this theory sheds no further light on the superfetation puzzle.
7One Baby Will Be Pre Or Post Mature
By its very nature, a superfetation pregnancy presents a problem over due dates. For example, imagine a woman is pregnant, and her baby is due on August 1st. The same woman then becomes pregnant again six weeks later and her second baby is due on September 12th. If you deliver both babies on the due date of the first, then baby number two will be six weeks early. If you leave baby two in to mature a bit longer, you place baby one in a dangerous position because the placenta cannot support them forever.
Therefore, assuming both mother and babies are otherwise healthy, and the pregnancy is otherwise uneventful, the biggest hurdle for the parents and their medical team will be deciding when the optimal delivery date will be - assuming the babies come on the same day of course.
6There May Be More Cases We Don’t Know About
The first well-documented case was from 1960 when Mary Tress of Baltimore gave birth to what the medical team first thought were regular twins. The boys Anthony John and Mark Francis were delivered by Dr. Paul C Weinberg of Mt. Sinai hospital. Upon examining the boys, Dr. Weinberg thought Anthony, who was born first, looked premature. Dr. Weinberg did X-rays of the kid's thigh bones, the results of which showed a disparity in bone age. Anthony was actually two months premature and his brother Frank, who was technically younger because he was born second, was full term.
There may be other cases where twins are not, technically twins at all but the result of superfetation. However, because they may not be obvious, the medical teams would have no reason to suspect and investigate if it were the case.
5Discovered During Ultrasounds
Other, more modern, cases have been identified through the use of ultrasound. An article in the medical literature speaks of a case where a woman went to an obstetrics clinic because of vaginal bleeding in her first trimester. The mom-to-be was found to have two sacs with a placenta for each and a live fetus in each one, but the fetuses were clearly at very different stages of development. One was almost 12 weeks gestation and the second was approaching eight weeks. She was monitored throughout her pregnancy, and the babies maintained a four-week difference in development.
A healthy baby girl was born and was concluded to be at 39 weeks while her healthy brother was delivered at the same time and was confirmed to be 35 weeks.
Without the ultrasound, this may have just been put down as another case of twins where one is smaller than the other.
4It Can Only Be Confirmed After The Birth
During each suspected incidence of superfetation, there has been extensive monitoring, and testing carried out during the pregnancy to exclude other causes of growth disparity. To begin with, sometimes twins are just as different genetically as any other sibling, and one might just be bigger than the other.
Multiple genetic anomalies can cause growth retardation as can many other medical conditions in both the baby and the mother. Twin to twin transfusion syndrome causes one twin to consume a much greater proportion of the resources available than the other. This can cause one twin to be starved of the energy it needs to grow and can be fatal if not discovered in time for the medical team to intervene.
Of course, some condition can cause excessive growth, and it may be that one isn’t much smaller than it should be but one is much larger than it should be.
These causes of growth disparity can only be ruled out 100% through testing after the birth.
3Fertility Treatment May Or May Not Play A Role
Another well documented modern day case involved a woman undergoing fertility treatment. The woman had three embryos implanted at once and a follow-up ultrasound two of the embryos had implanted, and the third embryo failed to develop into a fetus. The woman and her partner were told they were expecting twins.
Five months later, during another ultrasound, it was discovered that the woman was pregnant with three babies, the two implanted twins and a third fetus that was approximately three weeks younger than its siblings.
The doctors concluded that after the successful implantation of the two IVF embryos, the woman had ovulated less than a month later and became pregnant with a third child who successfully implanted and the couple had three healthy children.
It is thought that the infertility treatment the mother had undergone had over stimulated her ovaries and they basically went into overdrive.
2It Doesn’t Harm The Babies Or Mother
Shelly Hutchins had been diagnosed with endometriosis and never expected to have children, and when she and her husband had their first son, they assumed he would be an only child. When Hutchins became pregnant again, she was pretty big at eight weeks, and a scan showed she was carrying twins. A later scan raised questions because although on baby had clearly visible arms and legs the other did not and it appeared to be much more under developed than its sibling.
Scans every two weeks monitored the situation, and the same disparity in growth remained until a healthy son and daughter were born at 37 weeks. However, their daughter was much smaller and needed to spend two weeks in the NICU, and her development continued to lag behind her brother.
At the age of seven months, after every test imaginable the doctors said the babies were a case of superfetation and that their daughter had been eight weeks premature.
In this and every other documented case, apart from regular issues associated with prematurity the babies and mother had been perfectly healthy through pregnancy birth and beyond.
1You Don’t Need To Worry About It
The chances of you becoming pregnant again while you are already pregnant are so remote it is almost inconceivable (pun intended). Of all of the pregnancies that have occurred since 1960 across the entire world, only 10 cases of superfetation have been reported, and one or two of them are rather suspect because of a limited amount of documentation.
Looking at average statistics around pregnancy, only 30% of couples trying to have a baby become pregnant in the first cycle and about 63% of healthy fertile women in their early thirties will become pregnant within a year of trying.
In 2008, an estimated 6,578,000 pregnancies in the US resulted in 4,248,000 live births, 1,212,000 induced abortions, and 1,118,000 fetal losses. Of those, not one was even suspected to be a superfetation pregnancy.
So relax, if you’re pregnant, realistically you aren’t going to put another bun in the oven while that one is baking.
Sources: Scientific American, The British Medical Journal, Time Magazine, onlinelibrary.wiley.com, Ultrasound in Obstetrics and Gynecology 14, 219-221, Nature Communications 1:78 (2010), The Journal of Pediatrics 147:2, 254-255 (2005), Fertility and Sterility 68:3, 538-541 (1997), Biological Reviews, CDC
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