My sister is pregnant

Jackie • 24 y/o Jake G 💍 mama to LNG 4/21/17 👶💙. SAHM . Idgaf about your opinion. It doesn't phase me🙌I say whats on my mind. Gtfo my page if your a hater. I don't need that type of drama. Stay blessed . 🙏🏻
So my sister is 17 years old and she is 18 weeks pregnant . She still has not found out the sex of the baby... Why is it that most women who are less then 18 weeks find out the sex of their baby and my sister does not. ?  😡 does anyone know why this is?