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I'm currently in the middle of writing a story, and one of the story elements threw up a question for me. In this story, there are two siblings, who are only one or two months apart in age. As they grow older, they start to question how this age difference is even possible. How could their mother have had another child after just a month of giving birth to the first child? They suspect that they're not related and don't actually have the same mother, and it turns out that it's true.

I want to know if that suspicion is justified. I began to think of ways two siblings can be less than the usual nine months apart in age and yet still be related to each other. One idea I had was of twins, one of whom is born earlier, while the other had to stay inside the womb for another month for whatever medical reason.

Basically, my question is: can you have siblings (with the same mother and father), who are less than nine months apart in age? And if yes, how?

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    $\begingroup$ If you are going into science fiction, you can look into the Twin paradox – both born at the same time, but one aged faster. Works also with "normal" siblings. $\endgroup$ – Paŭlo Ebermann Apr 14 '18 at 20:11
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    $\begingroup$ They could be 24 weeks (maybe 22 or 23 weeks) apart if the younger one was born prematurely. $\endgroup$ – John B. Lambe Apr 14 '18 at 23:44
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    $\begingroup$ One sibling could be born to a surrogate mother using DNA from the same mother and father as the other twin. $\endgroup$ – niemiro Apr 15 '18 at 7:26
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    $\begingroup$ Maybe there was a calendar reform between the two births and they forgot about that? $\endgroup$ – Hagen von Eitzen Apr 15 '18 at 10:54
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    $\begingroup$ If this was the literature SE I would ask how these people know they're only 2 months apart, or why anyone would try to set them up as siblings without lying about their birth dates. Fraternal twins would be a very likely option if they are between 1 and 4 years old, and adding an arbitrary year to one characters fake age would be equally easy beyond this range. A typical baby only grows 2 or 3 inches in their first two months... it wouldn't be hard to tell them they're twins. $\endgroup$ – Gorchestopher H Apr 16 '18 at 17:11
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What do you mean by siblings?

If by siblings, you accept cases of individuals having the same father but not the same mother, then of course, it is possible! Below, I will assume you are referring to full siblings (eventually twins).

Age gap between twins

According to the huffingtonpost, there is a case of two twins that were born 87 days apart. On average, the age gap between two twins is rather of the order of 15 minutes (Rayburn et al., 1984).

Cousins that "look like" full sibs

Imagine family A has a pair of homozygotic twin daughters. Family B has a pair of homozygotic twin sons. If the sons of family B mate with the daughters of family A, then the offspring will be cousins but will have a relatedness of two full siblings.

One can of course extend this kind of crazy scenario to any number of generations apart.

Superfetation

I discovered the concept of superfetation in @froimovi's answer. From wikipedia

Superfetation (also spelled superfoetation and superfœtation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal. It is not believed that it occurs naturally in humans. There have been 10 reported cases of possible superfetation in humans.

As @1006a rightly pointed, the two babies might well be delivered during the same labour though. If they are delivered during the same labour, then they will have the same age counting from birth but different age counting from the time of fertilization.

Uterus didelphys

See @Bakuriu's answer!

Human intervention

Other alternatives would require some human intervention. For example, fertilization could have happened in vitro and the eggs were implanted in two different wombs. One womb could be a surrogate mother and the other one could be either another surrogate mother or the woman who actually donated the ovules.

Plenty of cloning techniques could yield two full siblings to be less than two months apart. Also, techniques of ex utero pregnancy could be used (although I am not sure we have the technology ready for that). Note that human cloning is illegal in many countries.

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    $\begingroup$ Obviously I meant two siblings who have the same mother and father, but I added the clarification anyway. I was mainly looking for natural ways on how this could happen. The cousin one seems like cheating, though. :P $\endgroup$ – noClue Apr 14 '18 at 20:10
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    $\begingroup$ "Note that human cloning is illegal in many countries.", the plot thickens. $\endgroup$ – Juzer Ali Apr 16 '18 at 11:03
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    $\begingroup$ @froimovi For better or worse, the stackexchange idea is to write the best answer. If you can write a better answer by combining the best two answers then that's fair game. The point is simply to help people as best we can. $\endgroup$ – David Mulder Apr 17 '18 at 7:00
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    $\begingroup$ One of my friends was adopted. When she found her biological brother, it turned out they were born like 3 ~ 5 months apart (sorry, don't remember exact number), but genetic tests strongly suggests they are full siblings. Go figure. @noClue if you just left it as a mystery in your story, it will be good enough for me. $\endgroup$ – Mołot Apr 17 '18 at 12:55
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Yes, this can happen in non-twins. There is a rare phenomenon that can occur in humans (and some other animals), called 'Superfetation'. This is when some time during pregnancy, a woman has a second oocyte fertilized and implanted. This event is an entirely separate conception, and results in two fetuses of slightly different ages in the womb simultaneously. There are only ten cases of this in medical literature. Here is a link to the abstract of a review on the subject. Perhaps you can discuss this possibility in your story. : )

https://www.ncbi.nlm.nih.gov/pubmed/18667283

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  • $\begingroup$ Interesting. If i could pick two answers, I would. Have an upvote instead. $\endgroup$ – noClue Apr 15 '18 at 1:59
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    $\begingroup$ But note that both babies would likely be delivered during the same labor, so the birthdates would be the same. This is probably the biggest danger of superfetation—the younger "twin" may be premature, even though the overall pregnancy is full term. $\endgroup$ – 1006a Apr 15 '18 at 5:40
  • $\begingroup$ @1006a But the first could be delivered through c-section to avoid this problem. $\endgroup$ – Loren Pechtel Apr 18 '18 at 0:23
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I have known kids who were 7 months apart in age - the younger one was a premature birth. No human intervention or other special circumstances required.

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  • $\begingroup$ This is the simplest and most straightforward answer. I'm sure this happens more often than many people would think. $\endgroup$ – user30881 Apr 16 '18 at 11:31
  • $\begingroup$ @Cascabel Yes months...7 weeks would be incredibly far from viability. $\endgroup$ – Bryan Krause Apr 16 '18 at 16:12
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    $\begingroup$ @BryanKrause I simply missed "younger" and thought this was talking about twins born at different times as many other answers were (in which case a 7 month gap is not particularly feasible). $\endgroup$ – Cascabel Apr 16 '18 at 16:19
  • $\begingroup$ I'm not sure where pojoguy lives, but I can confirm that I've also met a mother of two boys who were born (from her womb) 7 months apart. $\endgroup$ – dotancohen Apr 17 '18 at 18:54
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It's quite possible. Consider a couple who has problems conceiving children, so they get a surrogate mother who carries the couple's child. And six months later, with the psychological pressure gone, the couple conceives a child naturally. So you can have natural siblings, almost any time apart.

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  • $\begingroup$ that's a genius answer. $\endgroup$ – froimovi Apr 16 '18 at 4:54
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    $\begingroup$ Why posting an answer that has already been suggested in my answer (see under Human intervention)? $\endgroup$ – Remi.b Apr 16 '18 at 16:04
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Women with uterus didelphys have two different uteruses, and hence can be pregnant with two sons at the same time. The time of fertilization for the two uteruses can be different.

The wikipedia page also mentions cases of different birth dates for the children. There was a case where the births were 72 days apart.

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    $\begingroup$ Daughters, too, I would guess. $\endgroup$ – Llewellyn Apr 16 '18 at 20:23
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    $\begingroup$ @Llewellyn I use the term son as a neutral, since child does not imply the fact that it is the offspring of the mother and offspring sounds demeaning to me. $\endgroup$ – Bakuriu Apr 17 '18 at 18:40
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    $\begingroup$ @Bakuriu I'm guessing this is a language barrier. :) In English, son simply means "male child", daughter means "female child", and child just means "someone's son or daughter". Other languages might use different words when talking about a woman's child or a man's child, but in English the parent's gender doesn't matter. (I hope I haven't offended you. I certainly didn't mean to, but I don't know how to explain it differently either. I fully agree that offspring would be a bad choice.) $\endgroup$ – Llewellyn Apr 17 '18 at 18:53
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    $\begingroup$ @Llewellyn I can't speak for Bakuriu, but I think you've misunderstood them. As a native English speaker, I totally agree that 'son' carries much more reference to belonging to a 'parent' in the sentence than the word 'child' does. Additionally, it's definitely correct in English to use a male pronoun when the antecedental gender is indefinite. (Like you, I'm not trying to be argumentative; I just feel like there's been a miscommunication here) $\endgroup$ – forresthopkinsa Apr 17 '18 at 21:22
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    $\begingroup$ @forresthopkinsa “Son” is not a pronoun. $\endgroup$ – Emil Jeřábek Apr 18 '18 at 8:50
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  • Surrogacy. Full siblings can be born at any age separation at all.
  • Medical intervention. I'm sure I've heard of one (non-identical) twin being delivered early by C-section for the purposes of surgery, while the other was left intact and delivered later.
  • If we're only looking at genetic siblings.... If identical twins both partner with identical twins, any children conceived will be legally cousins but genetically siblings.
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