¶ Twins and Chimerism
Mares can sometimes carry twins, but this can be risky for both the mare and foals. If the vet discovers that your mare is carrying twins, you will have some risks and potential rewards to weigh, before deciding whether or not you should allow the mare to carry both.
¶ Chance of twins
Twinning will occur in about 1 out of 100 pregnancies (1%), and both identical and fraternal twins can happen. About 1 out of 10 twin cases will be identical, which means that one embryo split into two early in gestation. The other 9 out of 10 cases will be fraternal twins, which happens when the mare ovulates twice, and both oocytes (eggs) are fertilized.
How will you know that your mare is carrying twins? The pregnancy notification will let you know, and you will see two foal images on the mare's breeding tab.
¶ Risks and rewards
When you find out that your mare is carrying twins, you will have an option to terminate one (or both) embryos within the first trimester. Obviously, the reward of keeping a twin pregnancy is getting two foals from one gestation. However, there are a number of risks you should be aware of:
Guaranteed risks:
- Condition loss. Your mare will need a lot of calories during her 2nd and 3rd trimesters, a total Condition loss of -2.5, which is more than any feed can compensate for. As a result, your mare will lose some Condition during the pregnancy. Further, once the foals are born, she will need to produce more milk to feed both, losing -2 Condition. Concentrated feed will help her not lose weight during nursing, but she won't be able to do anything else without losing weight.
- Low Condition foals. A mare's uterus really isn't meant to hold two foals, and as a result, if she is carrying two, each one will receive less nutrients than a normal single pregnancy. Twins will then be born with lower Condition than normal.
Potential risks: (not guaranteed to happen)
- Embryo loss. The strain of providing space and nutrient for two embryos may cause one or both embryos to be lost during early gestation. The risk is small, but can occur at any time up until the 5th month of gestation. If the mare loses one embryo, then the pregnancy will continue like a normal singleton pregnancy, and none of these risks will apply any longer.
- Chance of chimerism. If one embryo is lost, there is a chance that the surviving embryo will absorb it, creating a chimera. See below for more information on what a chimera is.
- Chance of injury to the mare. At foaling, there will be a 20% chance that the mare will experience a peritoneal laceration. If this occurs, you will not be able to re-breed her until the laceration heals, which usually takes about 4 game months.
¶ If you do not want these risks
Similar to a regular pregnancy, you may terminate the pregnancy during the first trimester, however you will have the choice of terminating one or both embryos.
¶ Chimerism
Chimeras are a single individual (in this case a horse) which contains cells or tissues from two or more horses. Chimerism will occur when a twin is lost during gestation, and the cells are absorbed by the surviving twin. This is a rare occurrence, happening in only 10% of fraternal twin pregnancies where one embryo is lost.
How will you know if your foal is a chimera? Well, usually they will be easy to tell because they will have patches of a different color on their body where the other foal’s cells were absorbed. Something like this:
Sometimes they will also look brindle, depending on where the cells where absorbed!
What will happen if you try to breed a chimera? Well, in real life, there is a chance that the germ cells (the cells which reproduce) are chimeric. If that is the case, the horse can more or less reproduce as if they were two horses. Some foals would get sire A’s genetics, some would get sire B’s, basically. But, if the chimera didn’t get germ cells from their twin, and instead just got somatic cells, then they only reproduce from their own genetics. So for them, they just look cool, and it has no effect on the foals.
For the game, chimeras will only be somatic. So, they will just look pretty.