Yes, sorry, July 12. I counted one too many weeks back.
I agree with Annie. I you can afford to take the more expensive test now, that would either put his mind at ease, or give him a chance to think about how to proceed if he is the father.
You can get a DNA test from Ravgen or the DDC in the UK just as well as in the U.S.
Frankly, if the woman will do a prenatal DNA test, it would save everyone 18 weeks of telephone calls and stress to just do it. Your boyfriend is being an ostrich with its head in the sand not to even consider testing before the baby comes. It could be done next week. (Don't tell a woman who is calling to demand money that I said so, but in your boyfriend's shoes I would even be willing to pay for it all by myself if she would do it, just to get her off the phone and off your backs. Peace of mind is worth a lot of money.)
If the woman acts unwilling to do a prenatal DNA test now, that is suggestive that she has either lost her mind (and pregnant women write in to this community all the time that simply cannot think straight, due to their anxiety), or she is aware the baby might be her boyfriend's but wants to get what she can from the situation in the meantime. I don't like to suggest this is a scam, because as the dates line up in her story, the odds are at least 50/50 that the baby is from your boyfriend, and if so, you, she and he are going to need to be on good terms for the next many years because a child has to be raised without drama. And even though it's stressing you and is an annoying situation, keep in mind that even good, sane women do panic when their world shifts entirely over paternity while they are pregnant. Try to treat her as though everything she says is correct (except for the demands for money) and work with her even if she has limited understanding of what a 22nd-week ultrasound proves.
Regarding that point, you could suggest to her that she ask her doctor to explain to her how accurate a 22nd-week ultrasound is for determining the exact conception date of a baby. Unless her doctor dozed off during the part of medical school where they talked about ultrasounds, he or she will tell the woman that it cannot be used to pinpoint a conception date or determine paternity.
When she demands money, the answer is of course, "When we get a DNA test from a lab approved by the courts for determining paternity, and it shows he is the dad, we'll talk about child support." If she is being a pest, talk to a lawyer soon, not after a test determines paternity. She should at least be sent a proper legal letter telling your boyfriend's position (about testing having to come before any money) and asking her to stop being so constant with the phone calls. This kind of harassment has a legal remedy, but you shouldn't go that far without at least communicating appropriately through a lawyer that he is willing to do the right thing if it turns out that he's the dad, but that this has to happen first.
In the U.S., although prenatal testing at the top labs is accurate, it's still new enough that the courts only accept a DNA test that is done after the baby comes as proof of paternity. If your boyfriend does a test through Ravgen or the DDC it would be reliable, and will give you all 18 weeks of clarity that you won't have as things stand now. But if British courts are like US courts, he will probably be asked by the courts to do a DNA test once the baby comes. That's a great safety for him. Only when that post-natal test comes out positive for him, should he be talking about financial support for the child.
I know this is irritating, stressful and shocking. But please also, keep in mind that she has had a shock; her world has changed entirely and she is freaked out about it. And whether she's honest and stupid, or not being quite honest, she just might be in a pretty desperate situation. That should help you muster a modicum of sympathy, or at least courtesy, when she calls. And at the worst, change your phone number.
Short answer is, her boyfriend is not ruled out, the 22nd-week ultrasound is too late to be useful to rule him out. But your boyfriend is certainly not ruled out.
The woman is saying she didn't have sex with her regular boyfriend early in July, so that either she got pregnant from the sex on July 9 or she got pregnant from the sex a week and a few days later, say, the 19th or the 20th? And she is taking a 22-week ultrasound as evidence it's not from her boyfriend?
For a due date of April 4, the estimated conception date is July 12, not July 5. However, as
RockRose has said, a due date given in the 22nd week of pregnancy is not precise enough to point exactly to one of those sexual events as producing the pregnancy.
Using an ultrasound to try to determine a conception date gets less and less pinpoint over time because some babies grow faster and some grow slower than average. An ultrasound in the 7th week might have a margin for error of only a day or two, but by the time the woman is at her 40th week, trying to use it to guess at the baby's conception by looking at its development has a margin for error of +/- 3 weeks. The margin for error in the 22nd week will easily cover the time frame that both guys had sex with the woman.
In a civilized world, the pregnant woman, her boyfriend, and your boyfriend would contact Ravgen or the DDC, the two best labs in the world for prenatal DNA testing, and get a DNA test done now. (Both companies have relationships with labs all over the world to take the samples.) Prenatal DNA tests are done with a blood draw from the woman (her arm) and cheek swabs from both men. Either the three can go together to the lab if nothing is a secret to either guy about the situation, or she should go with each guy. (She needs to be certain the guys are not sending someone else in their place, and the guys need to see their swabs handed off to a neutral third party, not just to her.) And the three of them should split the cost of the test three ways. These tests are expensive, running from $1,600 to $2,000; given that these are adults who all went to bed of their own free will, in all fairness each person should bear a third of the cost. Then they will know in a week or ten days that one of the guys is not the dad, and one is the dad. She should understand that there is still a chance that her boyfriend is the dad, and this would be the way to find out.
Second best would be for just your boyfriend and the woman to do a prenatal test (without her boyfriend also doing a swab) and split the cost two ways. This is not as good as all three testing, since if he gets a negative, sometimes for some women this is not enough ("what if the lab is -- WRONG!?!?!?!") and sheez, your boyfriend and you don't want to have to field anxious phone calls from her for 18 more weeks obsessing that his negative might not have been a negative. If they all three test, one guy gets a no and the other guy gets a yes -- their test results act as a check on each other's test. I hope the situation here is calm enough that testing with all three people can be done.
I recommend going the route of doing a prenatal DNA test (versus a post-natal test, which are about ten times cheaper) because then you all know now. There would be time to plan and to think of what to do. But if it is simply not possible to manage even a third of the cost of prenatal testing, or if anyone point-blank refuses, the option would be to test once the baby is born. DNA tests can be done at the hospital as soon as the baby comes.
Of course, part of going through this is you and your boyfriend talking about what will happen if your boyfriend does turn out to be the father. At that point he should talk to a lawyer that does family law. The parental connection can be severed but only under certain limited circumstances (I believe the only case is if the child is being adopted, but don't take my word for it, ask the lawyer). Or, your boyfriend might decide he wants involvement, since this would be his biological child. (This of course will mean you yourself have some thinking to do about what that means in your relationship.) The lawyer can also go into what your boyfriend might expect if the woman seeks child support from him.
I'm sorry your boyfriend has to learn the hard way that part of being a modern male with access to free sex is that sometimes you have to take a DNA test and possibly face the music. Both of you should do your best to keep up a constructive relationship with the woman, she's been going along thinking everything was fine and now she has found out there is a big, huge question, and she thinks it changes everything she has been expecting and everything she has told her boyfriend. You all have some joint problem-solving to do now, and if the test comes out the way we might hope it doesn't, you'll have lots of need for joint problem-solving in the future. She is not wrong to be worried that he is the dad, but she is wrong in her assumption that her boyfriend doesn't have a chance. It is well worth testing now, to know what everyone is facing.
Her irregular periods aren't being taken into account here - this pregnancy hasn't been dated by when she had a period, but rather, the apparent gestational age of the baby. So irregular periods won't be a factor.
Yes, there's a chance the scan is so far off that her boyfriend could still very well be the father. But there are two other pieces of information that work in the other direction. If she's been having regular unprotected sex with her boyfriend and never got pregnant, that tilts the likelihood in the direction of your boyfriend being the father. Additionally, when women are ovulating (fertile) they often have greatly increased libido, which can lead to behaviors like one night stands and taking foolish chances with birth control. That's why you hear so many teen girls get pregnant on the night they lost their virginity - because the fact that they were fertile was what made them cross over the threshold into having sex.
What does this woman want to do - do you know? Does she want child support, or does she also want your boyfriend to have a parental relationship with the child? What would your boyfriend want?
Since the early scan didn't "date" the pregnancy, all we have to go by is the due date they've assigned the baby which can be off by a couple weeks sometimes. When you count back 38 weeks from April 4, that's when she is most likely to have conceived, and that date is July 5. From those dates, it's VERY likely your boyfriend is the father of the baby. Did she sleep with her actual partner one week later? And again a few days after that? If so he's still marginally in the window. If she didn't sleep with him until a week and a few days after your boyfriend's encounter with her, it gets less and less likely. This one will need a DNA test once the baby is born.