If you are going to be having unprotected sex together, you each need to protect yourselves individually — be positive you are both monogamous, and both get tested for the range of diseases that can affect you sexually.If either of you has any doubts, use condoms every time.So, this means when your doctor tells you you're four weeks pregnant, your baby's gestational age is actually two weeks.
If you are not getting the positive results you want (if you want) then you and your boyfriend should consult a physician for more specific advice and possibly testing — of you both. If you are trying to avoid pregnancy, then you are really pushing your luck. You should really, really use a reliable method every time you have sexual contact.
A lot depends on timing and randomness (also called luck).
As others have noted, pregnancy is more likely to occur if you have sex a few days before or after ovulation (usually about halfway between the start of one period and the next).
Well, since you already know that the girl is pregnant, it means the pregnancy is well underway and the uterus does not accept new competition entries anymore :)The second pregnancy will not happen, if this is what you're after.
Once the egg has implanted, any further ovulations cease; a few weeks later, the cervix is sealed with the mucus plug.