Can one twin be ectopic and the other one invisible to dr?

Yes, it is possible for one twin to be ectopic while the other is not. An ectopic pregnancy occurs when a fertilized egg implants outside the uterus, often in a fallopian tube, and may not always be detected immediately. The other twin could be developing normally within the uterus, making it difficult for doctors to identify the ectopic twin during initial examinations. This scenario, while rare, highlights the complexities of multiple pregnancies.