Ay, 18 weeks is still okay for long travel naman, but honestly super importante na mag-consult ka muna sa OB mo about your specific situation, baka may concerns siya based sa pregnancy mo na di namin alam. A few of us in my mom group did long flights around that time, pero iba-iba talaga ng experience, at mas safe if doc gives the go-ahead first, especially para sa peace of mind mo!