Twin pregnancy can be detected between 6-8 week of pregnancy or at the time of first ultrasound. There are two types of twins. One is identical twin in which one egg is fertilized and only one sac is present but zygote can divide in to 2 or more pieces each developing in to fetus. Such twins have same sex either male or female. Second type of twins are called fraternal twins in which more than one eggs are fertilized and number of sac correspond to number of fetuses. These twins can be of same or different sexes.
I was eight weeks pregnant when the first ultrasound confirmed that I was having twins. Most OBGYNs won't see you before eight weeks unless you're having problems. Whether or not your having identical twins or fraternal twins? DNA testing after birth is sometimes needed to be certain. It's not as cut and dry as some would like you to believe. It's possible for identicals to have separate sacs if the egg split early enough.