Many newly pregnant women wonder, "Is it twins?" Although twin pregnancies are more easily identified later in pregnancy, even at the time of the first missed period, or 4 weeks of pregnancy, there may be clues that more than one baby is on the way. Some signs are definitive, but others are only suggestive.
The gestational sac is the first sign of pregnancy seen on ultrasonography. The gestational sac looks like an oval-shaped dark hole when you look at the ultrasound picture. At 4 weeks, the gestational sac is often visible on transvaginal ultrasonography, and if you're having twins, there may be two sacs, if the twins are fraternal, or non-identical. Identical twins develop in the same sac, so it may not be obvious that there is more than one embryo until the end of the fourth week or into the fifth week, when the yolk sac--the early source of the embryo's nutrition--may first be seen. If identical twins are present, two yolk sacs will be seen within the gestational sac.
Human Chorionic Gonadotropin Levels
Levels of human chorionic gonadoptropin, or hCG, measure the amount of the hormone produced by the placenta. Women having twins generally, but not always, have higher hCG levels or faster rising hCG levels than those with singleton pregnancies. There is a wide range of normal hCG levels at 4 weeks. A high hCG level isn't a definite indicator of twins, but it may give you a clue -- especially if the pregnancy was achieved through assisted reproductive technology, which is associated with a much higher rate of multiple births.
Women having twins often have more nausea, more frequent urination, sorer breasts and a higher level of fatigue than those carrying singletons because of the high level of pregnancy hormones such as estradiol and progesterone. Even at 4 weeks, when the first period is just being missed, women with twins may already have symptoms of early pregnancy.