See Gemara Nidda 31a about conjugal relationship during pregnancy:
תנו רבנן שלשה חדשים הראשונים תשמיש קשה לאשה וגם קשה לולד אמצעיים קשה לאשה ויפה לולד אחרונים יפה לאשה ויפה לולד שמתוך כך נמצא הולד מלובן ומזורז תנא המשמש מטתו ליום תשעים כאילו שופך דמים מנא ידע אלא אמר אביי משמש והולך (תהילים קטז) ושומר פתאים ה׳
(Soncino translation)
Our Rabbis taught: During the first three months marital intercourse is injurious to the woman and it is also injurious to the child. During the middle ones it is injurious to the woman but beneficial for the child. During the last months it is beneficial for both the woman and the child, since on account of it the child becomes well-formed and of strong vitality. One taught: He who indulges in marital intercourse on the ninetieth day is as though he had shed blood. But whence could one know this? - Rather, said Abaye, one carries on marital intercourse in the usual manner and the Lord preserveth the simple.
But primarily your reflex must be to ask a gynecologist as said @msh210 . In Halacha every issue which us linked to health should be discussed with a qualified physician.