I am so sorry for your loss. Many people don’t seem to realize that you CAN love a child that you have only known for a short time. I have had 4 losses, and each one was a child I loved and grieved for.
As an OBGYN physician assistant, I can tell you it is not medically necessary to wait after a miscarriage. It is considered old school advice - especially if your wife did not have a D&C. (If that is the case, than like Jennie mentioned, you should wait 6 weeks before resuming intercourse). The other exception is if you have a molar pregnancy. Then you need to wait 6 months to a year or more.
You and your wife need to decide when she is ready, emotionally and physically. Some women want to wait longer than others do. My OB told me I SHOULD wait one cycle after my first loss, but then in the next breath he told me that his wife conceived immediately after her miscarriage, and that you are more fertile after a loss. We decided not to wait and leave it in God’s hands. We did conceive again almost immediately, but also miscarried again. (It was later determined that I had a cause for recurrent pregnancy loss, so please do not think that if you conceive soon after a loss you will automatically have another miscarriage).
If you do decide to wait, use the first day of bleeding from the miscarriage as cycle day number 1. Your wife MAY or MAY NOT have a “normal” cycle. Some women have a longer cycle after a miscarriage, up to 36-40+ days versus their regular 28 day cycle. Some go right back to their regular routine. But like a PP mentioned, be sure to pay close attention to her fertility signs (cervical mucous especially) in case things are atypical. My cycles were always very erratic after my miscarriages, inlcuding longer, heavier bleeding, cramping, longer cycle length. I decided just to forgo any method of trying to chart things and let whatever happen happen. But like I said, some women do go back to their normal pattern.
I will say some prayers tonight for you, your wife, and your baby. I hope you both are able to find some comfort soon.
J