Q: I am 18 days post ovulation (DPO) and my serum hCG is 556. Could this mean I am having twins?

A serum hCG of 556 mIU/mL at 18 days post ovulation (DPO) is normal both for one or two fetuses. You could have twins, but you can't really tell from the serum hCG value alone. At 18 DPO the serum hCG for singletons is normally between about 70 and 750 mIU/mL, and for twins, it can be between 200 and about 1750 mIU/ml.

Are you always carrying twins when hCG increases too fast?

In most pregnancies, the normal hCG rise is at least 60% over 48 hours. In twins, it may rise much more, but it's not possible to diagnose a twin pregnancy just from the hCG levels alone. The hCG can rise manyfold even without twins.

How does hCG normally increase?

1. In most normal pregnancies at hCG levels below 1,200 mIU/ml, the hCG level usually doubles every 48-72 hours, and it normally increases by at least 60% every two days.
2. In early pregnancy, a 48-hour increase of hCG by 35% can still be considered normal.
3. As your pregnancy progresses, the hCG level increase slows down significantly.
4. Between 1,200 and 6,000 mIU/ml serum, the hCG level usually takes 72-96 hours to double.
5. Above 6,000 mIU/ml, the hCG levels often take over four or more days to double.
6. In general, when the hCG level reaches 7200 mIU/ml, a yolk sac should be seen.
7. At an hCG level greater than 10,800 mIU/ml, there should be a visible embryo with a heartbeat.

There is no sufficient scientific evidence that with twins there is always a faster-than-usual rise in hCG. Normal hCG values can vary up to 20 times in normal pregnancies. Variations in hCG increases are not necessarily a sign that the pregnancy is abnormal or that there are two or more fetuses. 

Chart showing average serum hCG levels for twins and singletons 

	Twins	Single
DPO	Average (mIU/ml)	Average (mIU/ml)
14	68	48
15	88	59
16	146	95
17	154	132
18	360	292
19	499	303
21	963	1061

Read more:
hCG Levels by Week
Normal hCG Levels In Early Pregnancy
hCG Levels Chart And Calculator