A miscarriage can have multiple causes:
Miscarriages are more likely during a first pregnancy: the uterus isn't completely ready yet but it's not a cause for concern: the following pregnancy has every chance of success.
This represents 90% of early spontaneous miscarriages. When the foetus has a genetic abnormality (trisomy, monosomy, etc), the woman's body rejects the embryo, as if it were a "foreign" body. There is no reason for the following pregnancy to be affected.
The quality of the man's sperm can sometimes be the cause of recurrent miscarriages. Get a sperm test to be sure.
If successive miscarriages occur after seven or eight weeks of pregnancy, hormones could be the reason. This can be cured wth a prescription of progesterone until the 12th week of pregnancy, before the placenta takes over.
Uterine (womb) abnormality
Miscarriages can also be caused by a deformity of the uterus or cervix. Sometimes, under pressure from the uterus, the woman's cervix opens and the foetus passes out.
If the mother of the pregnant woman was prescribed Diethylstilboestrol (DES), one doesn't need to look much further: this synthetic form of oestrogen, designed to prevent miscarriages, is often the cause of miscarriages.
For those who haven't been exposed to DES, a weak cervix can be treated with cervical stitching
(cerclage), whereby stitches are placed in the uterus to keep the cervix closed during pregnancy.
Certain infections or sexually transmitted diseases
may cause miscarriages but nothing is certain...
Once again, nothing is proven: just try to live a balanced lifestyle. Don't lose sight of the fact that, in the majority of cases, miscarriages are often due to external factors, and the large majority of miscarriages have no explanation.