Be sure when purchasing your jennet that you always have a prepurchase exam, not only a physical done but have a uterine biopsy and culture done to help make sure that she not only has no infections, but to help indicate if she can even get pregnant.  The next best thing is to purchase her pregnant with an ultrasound to prove that she is pregnant and not carrying twins.

Beware the jennet that was allowed to run with geldings while being open.  Gelding donkeys can and do breed jennets, they go through the entire motions but of course are shooting blanks.  This constant barrage from a gelding can contaminate the jennets reproductive tract and make her difficult to settle, if not sterile.  Only a uterine exam/biopsy can  attempt to show this.

Another thing to consider with your prospective brood jennet, especially if she is a maiden, is how well has she been socialized?   Does she think she is a human or has she been raised with  other donkeys.  It has often become the sad practice of weaning a foal early from it's mother, some people will even take them away as young as 3 months (I think it's criminal to wean a foal any younger than 5 or 6 months).   These foals are placed in homes where there are no other donkeys and then later on they are bred.  The potential is quite high for a jennet raised like this to reject her foal - she has learned no social skills to deal with motherhood  and is understandably confused and frightened.  Not everything is instinctive, learned social behavior is important.   If you want a jennet to grow up to be a good mother - she needs to be well socialized and hopefully has been around other mothers and foals as she grows.   I will often use my yearling and two year old jennets as babysitters when weaning  foals - they learn to put up with the baby shenanigans, and get a chance to practice mothering.  It's so cute to see the young babysitters importantly standing guard over a napping weanling.

Your breeding jennets should not be overly fat, but they should be in good flesh. They should also be physically mature before breeding.  This means that they should not be bred before their third birthday, better yet wait until they are four years of age for jackstock (they are not done growing for at least 6 years). As well as physically fit, your donkey should be mentally fit as well.  They should be content and relaxed and happy in their environment.  Stress could possibly inhibit conception.  Jennets usually cycle every 18-21 days, with their heats going from average of 2 to 8 days.  They normally ovulate near the end of their heat cycle. 

Generally jennets stop or slow their cycling during the winter.  Another interesting phenomena is that they seem to also stop or go silent during the hot summer months as well, making them more spring and fall breeders (in the Pacific Northwest anyway).  This could reflect a desert survival adaptation. In a hot desert, maybe foals born during the scorching summer months had no chance of survival.

People often want to bred a hinny from a jennet (the sire is a horse).  The fertility from this cross is less than the average for mule foals from mares. The hinny fetus, because it is part horse, could be heavier than a donkey foal, this makes it a harder pregnancy for the jennet.  It's not something I recommend for the jennets as it can break them down faster, as each pregnancy is a wear on their bodies.  Besides, in the case of mammoths, because of the low number of quality animals, it seems a waste to 'use up' those pregnancies on anything other than breeding mammoths.

Conformation Considerations for Jennets:

•  An even bite where front incisors and molars meet correctly. 
•  Straight sound legs all around, slight cow hocks could be forgiven if she moves out straight and cannon bones are parallel when standing.
•  Short strong back with smooth tie in to the croup
•  Nice long neck attached to a well laid back shoulder, with adequate width and depth of chest 
•  Head should blend with body size, eyes large and expressive - ears long and upright
• Jennet should appear feminine, with more refinement than a jack.
•  Powerful deep hind end, wide pelvis, with fairly level or moderately sloped croup hip should near level in height to withers. 
•  Laid back and gentle disposition.
•  A jennet of perfect conformation is always the perfect color!