In years past, women often had no idea that they were carrying twins until the end of pregnancy or the actual childbirth. Since the development of better tests, most multiple pregnancies are now identified during the first or second trimester.

A fetal ultrasound can show whether there is more than one fetus in the uterus. If you have more than one fetus, you will have an ultrasound several times during the pregnancy to monitor fetal growth and amniotic fluid.

Sometimes, the first sign of a multiple pregnancy is from a test that was done for another reason. For example, a very high level of human chorionic gonadotropin (hCG), the "pregnancy test" hormone, can be a sign of multiple pregnancy.

Tests used to check for genetic disorders and birth defects

Fetuses in multiple pregnancies have an increased risk of genetic disorders and birth defects.

Test options include:

• Chorionic villus sampling (CVS). It uses a tiny piece of the placenta, taken by passing a thin tube through your vagina and cervix and into the uterus. The sampling and genetic testing are done between 10 and 12 weeks of pregnancy. After about 13 weeks, this procedure may be difficult to perform.
• Amniocentesis, which uses a small amount of amniotic fluid, taken by inserting a needle into your abdomen and uterus. The sampling and genetic testing are usually done between 15 and 20 weeks of pregnancy. (Amniocentesis is also sometimes used in the last trimester to see whether the fetuses' lungs are mature enough to breathe well after delivery.)

CVS and amniocentesis have the same slight miscarriage risk when used to test a multiple pregnancy. (For a pregnancy with one fetus, the CVS risk is a little higher.)⁵ You may want earlier CVS results if you have to make decisions about treating or continuing a pregnancy.

Tests used to monitor the fetuses and mother during a multiple pregnancy

• Blood pressure checks at every prenatal appointment are used to monitor you for high blood pressure or preeclampsia.
• Blood testing is used to check you for low iron (anemia). Anemia is a common problem for women with multiple pregnancy, because the fetuses use a great amount of the mother's iron stores.
• A urine test and urine culture can be used to screen you for a urinary tract infection (UTI).
• Transvaginal ultrasound may be used to check the length of your cervix. A short cervix is a sign of an increased risk of preterm labor.
• In the second trimester, you may have an oral glucose screen to check for gestational diabetes.
• Electronic fetal heart monitoring may be used before or during delivery, to make sure the fetuses are doing well.