How big is the baby measuring now? I remember with my son, the nurses assistant put me in for as being due in August but I had IVF so I knew exactly when conception had happened and she was off 2 weeks. Nobody really changed it until I had a scan and they seen the baby was further along and finally corrected my gestation time.
On another note, I'm on my 2nd pregnancy and already showing a little bump at 4 weeks. From what I've read the 2nd pregnancy you get bigger faster and way more pain because your muscles stretch and pull faster. Good luck with everything...ask for another scan for piece of mind.
At that stage twins develop the same rate as a singleton pregnancy it's only when they start fighting for room in your womb that they grow slower x