Heather Corinna replies:
Y'all need to remember that when anyone is doing the things that can cause pregnancy, pregnancy is always a possibility.
In other words: are you having direct genital-to-genital contact with someone who has a different reproductive kind of system than you do? If so, pregnancy is possible.
How possible it is depends on what method or methods of contraception were used and on how well they were used.
In this situation, you're talking about first using a condom, then using withdrawal for round two. In perfect use, condoms are 98% effective, and in typical use, they're 85% effective. Withdrawal is 96% effective in perfect use and is 73-82% effective in typical use.
With the condom, did you use it perfectly? That means a condom was on, from start to finish, for ALL or any direct genital-to-genital contact, that it was a condom within the expiry date, that you left room in the tip of the condom and pushed the air bubbles out, and that you also took it off correctly, with all ejaculate remaining in the condom. It's a bit of a tricky question to answer in this case where you're kind of talking about two different incidents of intercourse, but since you clearly didn't have it on for all contact in that given sexual session, I'm going to say no, it wasn't used perfectly, which could present around a 15% risk of pregnancy.
With the withdrawal, perfect use generally means not having a penis with ejaculate already on it, urinating after a previous ejaculation, and then being sure that with the intercourse, you withdraw your penis from her vagina well before you ejaculate. It doesn't sound like you did the first, so I'm going to say that with that one, you probably also were only at typical use, which presents a 27 - 18% risk.
If we look at them combined, though, the typical use combined rate of effectiveness is around 95%, which means there may have only been around a 5% risk. If your girlfriend is not comfortable with that level of potential risk, or the potentially higher levels of risk I talked about above, she can consider using emergency contraception.
I want to make sure it's clear that for birth control methods to work at preventing pregnancy, they have to be used both consistently and correctly: they need to always be used and need to be used properly. The vast majority of the time, when contraception fails, it's due to incorrect use or nonuse.
If you and/or your partner don't want to create a pregnancy but do want to have the kinds of sex that can, it's very important that whatever methods you're using to prevent pregnancy are used as best as they can be. If your method is condoms, that means using them all the time and every time to the letter: putting them on right before any contact every time, using lube as needed, leaving room in the tip, taking them off correctly. You can find clear instructions for how to use condoms here: Condom Basics: A User's Manual. If your method is withdrawal, that means not entering her body when there is fresh ejaculate on your penis, urinating first, and always withdrawing well before you'd ejaculate. If you use both properly, combined, you're actually looking at really excellent protection: over 99%. If you don't, then you're looking at the figures I gave you or even lower.
Don't forget, too, that pregnancy isn't the only physical risk there is: sexually transmitted infections are something you both need to think about, too. The rates of STIs in people 15-25 are higher than those for other age groups because so often, younger people figure pregnancy is the only thing to be concerned with, and that STIs happen to other people, not to them. With one out of every four of you in that age groups carrying an STI, we know that just isn't true. STIs are absolutely something for you guys to be concerned about, and that you need to protect yourselves and your partners from.
If you're going to go without condoms at all, but want to assure as best you can neither of you transmits or contracts an infection, you'll need to practice safer sex. That means that for the first six months you two are together, you use condoms, without fail or exception, for any oral, vaginal and/or anal sex. You also stay sexually exclusive for those six months, then both get a round of STI tests from your healthcare provider at the end of that time period. If you both have all negative results and both stay sexually exclusive after that, then you can ditch condoms if you want and be very unlikely to deal with an STI.
So you know for the next time, it's a great idea to talk with a sexual partner about all of these things in advance of sex. Talk about how you both feel about a potential pregnancy or STI, and make a plan you both agree on about what you're going to use to prevent those outcomes, choosing methods that give you both the level of protection you're both most comfortable with. Make an agreement to always stick to that plan, and for both of you to support the other in sticking to it. If at any time you find you don't feel capable of sticking with it, that's a good time to take a pass on sex. As great as sex can be, winding up with a pregnancy or an infection can make what was otherwise a great experience a really lousy one.
One last thing? Do remember that you're not the one who can get pregnant here: a female partner's level of risk in this is far higher than yours, and if things go wrong, she's going to be stuck with far bigger responsibilities than you are. Many STIs also present greater health risks to female-bodied people than to male-bodied folks. I think that given, it's only fair to figure that a male partner needs to really step it up to be sure he's being really supportive of contraception and safer sex, and also having a discussion with female partners if you're going to change the plan (as she should with you). In other words, did you take a pause after you removed that condom and both talk about if intercourse without a condom was really okay with both of you? If not, it's really important you do that. It's also important in something like that that you both know what your risks may be: if at any time you're about to do something where you don't know what risks it might pose, put it on hold. Go get that information, consider it, bring it to your partner to consider it, and only then, if you're both okay with that, do you go ahead and do that thing, okay?
In this case, both methods were also really up to you. If that feels like too much responsibility for you to manage, you can ask your female partner if she'd consider using a method that she's also responsible for, or again, put sex -- or the kinds of sex that present a risk of pregnancy -- on hold until you do feel capable of bearing this much responsibility.
Here are a few links to round this out, some of which you may want to share with your partner: