Critical foetal development occurs during the first trimester of pregnancy.
Dental implant placement is a surgical procedure that requires the use of local anaesthetic injections, X-rays and even bone grafting material. Post-surgical care includes the use of painkillers and antibiotics.
Most drugs are safe to use during pregnancy. Dental X-rays are safe during pregnancy (European guidelines on radiation protection in dental radiology) (https://ec.europa.eu/energy/sites/ener/files/documents/136.pdf).
However, to cater to cultural preferences and to minimize the risk of litigation, especially now that women tend to have children later with the help of assisted fertility procedures (IVF), most dentists prefer to defer non-essential dental treatment to later (in the 2nd or 3rd trimester) or after pregnancy.