Re: Animal litters often have a runt. Do human multiple births have a runt?

Scott,
Twins are never exactly equal, even when they are identical.  In animals 
that have bicornate (two-horned) uteri, these uteri are designed to 
handle multiple embryo implantations and in a lab animal like a mouse or 
rat or hamster there may be as many as a dozen and in some cases close to 
20 embryos that implant along the two horns of the uterus.  This large 
number of embryos is what ends up producing one or more runts because 
some of the egg/sperm combinations are not as genetically viable 
(healthy) as others.

If you actually go in weekly and look at the embryos and fetuses with surgery 
as they are developing during the gestation period in this kind of animal that 
has so many pups in a litter, you find that the number of implantation sites 
that start growing in the first week will be more than the number of embryos 
growing in the second week.  The number of embryos growing in the second week 
will be more than the number of fetuses growing in the third week and that 
number will normally be more than the number of live pups delivered at the end 
of the gestation.  What happens to the embryos and fetuses that are lost 
and why are they lost?  We call this loss resorption of the embryos or 
fetuses and the mother’s uterus just clears out (reabsorbs) the material 
of any embryo or fetus that can’t continue to live.

Now some of the embryos are good enough to stay alive but don’t keep up with 
the growth pace of the completely normal embryos and so they grow to full term 
(20 days for a mouse or 16 days for a hamster) and are delivered (born) but 
are small for gestational (pregnancy) age and may have problems even 
staying alive immediately after parturition (birth).  If this runt 
manages to stay alive after birth it may still have a hard time reaching 
full potential if it is a large litter and everybody is always fighting 
for a spot at one of the nipples.  Most mice, for example, have 8 nipples 
but might have 12 or more pups in a litter.  You can do the math and see 
that there will always be a few odd men out as some put it.

Now we come to humans and your question which brings me back to the point that 
no twins are exactly equal.  With humans, unlike mice and rats and 
so forth, the parents are very interested in each child and will 
carefully look at the babies and work very hard to be sure that everybody 
gets everything they need to grow big and strong.  If you ever get a 
chance to have a litter of mice or maybe hamsters to observe in your 
class you will notice that the mother never looks at her litter to give 
special preference to the runts so that they get all the milk they need.  
She is totally uninterested in the individuals and the pups can fight all 
they want to get to the milk, she doesn’t interfere at all.  If you make 
it, you live, and if you don’t make it, you die.  With humans we can plan ahead 
as well as evaluate the past and make adjustments so that both babies in a twin 
birth will do equally well even though they don’t start out exactly the same.  
If there is a physical defect in an organ, however, that does not allow the 
baby to grow normally, then that is a different story and might require medical 
attention if something can be done to improve the situation.

So yes, there can be runts in a human multiple birth group but generally there 
is no noticeable difference and so we don’t normally have a baby that would 
be called a runt when there are twins or triplets or more unless there is 
an obvious difference.  Of course the more babies a woman has with one 
pregnancy, the more chance there will be a runt.  Now get your teacher to 
buy a pair of hamsters and you will be able to see all this happen in 
your classroom.