Twin pregnancies

Twin pregnancies are becoming increasingly more common in Amercia secondary to women choosing to have children at a later age and the increased use of assisted reproductive technology to aid in achieving pregnancy.  Twins are either monozygotic (one egg fertilized by one sperm splitting into two eggs) or dizygotic (two eggs fertililzed by two different sperm).  Monozygotic twins are identical genetically where dizygotic are different genetically.  The frequency of dizygotic twins increase as a woman ages and has more children.
Twin pregnancies have more risks associated with them and require increased monitoring.  Twins have an increased risk for intrauterine growth restricion(small baby), premature delivery, congenital birth defects, abnormal placental development and stillbirth.  Because of these risks, a woman who has a twin pregnancy will need frequent doctor visits, ultrasounds and prenatal testing.
When a woman is diagnosed with a twin pregnancy, it is important to determine by ultrasound whether the twins have two placentas and two sacs, one placenta and two sacs, or one placenta and one sac.  The risks of the above mentioned complications are more common in pregnancies with one placenta and one sac.
It is also important to consider genetic testing for chromosomal abnormalities in twin pregnancies.  A woman has a double risk of chromosomal abnormalities with twins secondary to having two fetuses instead of one.  Screening tests include a first trimester ultrasound and blood test. Invasive diagnositic testing is also available in the first trimester by chorionic villus sampling.  (Please see my previous post on genetic screening in pregnancy for more detail regarding these tests and procedures). 
It is important for a woman with twins to obtain a detailed anatomy ultrasound of the fetuses between 18-22 weeks.  As twins have an increased risk of congential birth defects, it is important to try to detect these during this ultrasound.
As a woman progresses further in pregnancy it is important to monitor the fetal growth by ultrasound.  Twins tend to grow at the same rate of single pregnancies until 32 weeks and then have slower growth.  Twins can also have vascular connections in their placenta which enable one twin to grow larger than another.  It is important to monitor this because if one twin is larger than another by greater than 10 % weight, the smaller twin has an increased risk of stillbirth. 
It is also important to monitor the fetal well being during pregnancy by antepartum testing.  This involves monitoring the fetuses heart rate for approximately 30 minutes twice per week and checking the amniotic fluid once per week.  This testing reveals whether the uterus is still a good environment for the fetuses allowing them to grow and obtain adequate blood flow.  This testing will usually begin at 32 weeks and continue until delivery.
A woman has increased risk of medical complications associated with twin gestation as well.  She is more likely to develop gestational diabetes or pre-eclampsia (elevated blood pressure in pregnancy).  It is important to have regular doctor visits to monitor for these symptoms.
The average age for twin delivery is 35 weeks.  This delivery age would change depending on what type of twin gestation a woman has.  The route of delivery (vaginal versus cesarean) depends on a number of factors.  For a twin pregnancy consisting of one sac and one placenta, the optimal route of delivery will always be cesarean delivery usually around 32 weeks.  For twins with two sacs and one placenta, optimal delivery is usually between 36-37 weeks gestation.  The route of delivery depends on the presentation of the fetuses.  If both fetusus are vertex (head down), vaginal delivery is recommended.  If the first fetus is breech (bottom or feet down) cesarean delivery is recommended.  If the first fetus is vertex and the second fetus is breech, vaginal delivery is possible as long as the second fetus is not greater than 20 % of the first fetus's weight.  For twins with two sacs and two placentas, delivery is usually recommended at 38 weeks.  The route of delivery depends on the same factors listed above.
It is important to be under an obstetrician's care during pregnancy to facilitate the best pregnancy outcome possible.  Please read further for more info:
http://www.acog.org/~/media/For%20Patients/faq092.ashx