Note: The mail service will be listening for your connection on TCP port 25. But this information will not be important until you see some Perl code later in the chapter. The message that you prepare can only use alphanumeric characters. If you need to send binary information (like files), use the MIME protocol.
With my first set of twins we were able to tell at 16 weeks and with my second set at 14 weeks. It all just depends on their position. I was having ultrasounds every two weeks so there were alot of oppurtunities to look. It would not matter if they are different sizes. Helpful (8) Nice.
That being said, it IS possible to miss a twin for awhile. There was a girl here or on r/cautiousbb who found a twin at 21 weeks during her anatomy scan. Rare, but could happen. Bring your concerns up to your doctor but truthfully, I feel if they haven't seen 2 babies by the time your second trimester starts, you're probably just having one.