Having said all that, there are upsides too.  After all, if you were planning to have two kids anyway, then you only have to go through the whole pregnancy thing once (him through the stress and her through the birth, backaches, pains, hormones.)  Also don't forget that you will get double the enjoyment whilst they grow up together.

Medical implications:

Although multiple pregnancies are automatically classified as "high risk", the first thing to emphasize is that most women go through a twin pregnancy with no serious complications.  The problems of back pains and mood changes can be amplified a little but generally, all goes well.

However there are some factors that need to be remembered with multiple pregnancies.  The most important is that the nutritional intake of the mother needs to be increased but without excessive weight-gain (this should be discussed in detail with your doctor).   In general, most practitioners suggest an extra 250 to 350 calories per day (with higher levels of protein intake) as well as higher dosage vitamin supplements (but never simply double up on the standard vitamin/mineral intake for a single pregnancy, this can be harmful).

The obstetrician will almost certainly want to see the mother of a multiple pregnancy more times during the nine months than is usual for a single.  There will certainly also be more ultrasound sessions to attend so that progress of the development of both babies can be closely followed.

Under certain conditions when there is a concern over the efficiency of the placenta and the over-stressing of the womb, a woman carrying multiple babies may be ordered to take bed rest during the last few months of the pregnancy.  This can be a difficult thing to handle for both her and her partner. Luckily a long bed-rest is a rare occurrence whereas a short bed-rest over the last couple of weeks can be required often.

Implications for the delivery of multiples:

Many doctors presume that twins are at full term about two weeks before the standard due date.  Not all, but most multiple pregnancies arrive early.

An early delivery is mainly caused by the fact that multiple babies will stretch the womb quicker than a single child and the cervix may begin to loosen up earlier than would be expected.

If the twins are born very early (before 32 weeks after conception) then it is likely that they will need a little time in an incubator to help the lungs to finish full development.  If the birth is after 32 weeks from conception then just a few days in hospital is often all that is required.

Cesarean section delivery is an option that will be prepared in advance at the birth of multiples. If any complications are found with ultrasound in either of the twins (stressful heart rate or bad position) then a cesarean delivery is often the best way to ensure a satisfactory conclusion.  

In many locations around the world, a normal vaginal birth is encouraged as the preferred option.  There are good medical reasons for this but spare a thought for the poor mother that has a vaginal birth with the first baby then the second decides that it wants to change position and a cesarean is ordered for the second.

see also...

Introduction

The differences between identical and fraternal twins

Explanation of cellular division in twins

Frequently Asked Questions about twins

The implications of a multiple pregnancy

Possible complications in a multiple pregnancy