# Twins: at what point in gestation can sex by determined by US? **URL:** \ **Category:** In My Humble Opinion\ **Created:** [December 7, 2015, 4:31pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713 "2015-12-07T16:31:35Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [December 7, 2015, 4:31pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/1 "2015-12-07T16:31:35Z")
A woman I know has told everyone she is pregnant. Cool! A few weeks later, still not showing, she told everyone she is carrying twins. Double cool!! However, now she is telling everyone the twins are both boys according to ultrasound. Now everyone is starting to wonder. She _maybe_ has gained a few pounds, or else she is just wearing clothes that suggest this. Someone asked her due-date and she said she has a lifelong history of erratic periods so they can’t be sure. She is a very nice person, though a bit odd. So . . . what do you think? ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [December 7, 2015, 5:30pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/2 "2015-12-07T17:30:19Z")
Sex can be determined by ultrasound at about 4 or 5 months, it doesn’t matter if they’re twins except in that they may be hiding each other. Many women don’t really show much before the 6th month, specially for a first pregnancy or if they’re fat to start with. The erratic periods thing can perfectly well be true. ---
**Author:** ![Caffeine.addict](https://avatars.discourse-cdn.com/v4/letter/c/f475e1/32.png) [@Caffeine.addict](https://boards.straightdope.com/u/Caffeine.addict)\ **Post date:** [December 7, 2015, 5:47pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/3 "2015-12-07T17:47:05Z")
During both of my wife’s pregnancies, we found out when they did the nucal translucency test at around 12 to 13 weeks so at around 3 to four months. She wasn’t showing them much. ---
**Author:** ![Kent\_Clark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kent_clark/32/105_2.png) [@Kent\_Clark](https://boards.straightdope.com/u/Kent_Clark)\ **Post date:** [December 7, 2015, 5:51pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/4 "2015-12-07T17:51:11Z")
My wife had an ultrasound at 16 weeks. The ultrasound showed both were male, as weas confirmed by the amnio. FWIW, it’s a lot easier to mistake a male (who’s “hiding it” in the ultrasound) for a female, than vice versa. ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [December 7, 2015, 5:57pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/5 "2015-12-07T17:57:59Z")
> [@kunilou](#): > > FWIW, it’s a lot easier to mistake a male (who’s “hiding it” in the ultrasound) for a female, than vice versa. I’m picturing Dr. Nick from the Simpsons: “You’re having a daughter with eleven fingers! And one of them’s not on her hands!” ---
**Author:** ![delphica](https://avatars.discourse-cdn.com/v4/letter/d/e9a140/32.png) [@delphica](https://boards.straightdope.com/u/delphica)\ **Post date:** [December 7, 2015, 6:12pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/6 "2015-12-07T18:12:20Z")
I had a high risk pregnancy, so I was having frequent ultrasounds (to monitor a specific medical issue) early on, and so we knew it was “probably” a girl by about week 14, which is very early and not considered especially reliable. It was more of a conversational chat with the tech person about her best guess. If your friend is having US for a medical reason (I don’t know the protocol with twins), it might have come up. Usually, the US for the purpose of the parents learning the gender and making the fun announcement to friends and family (if they wish) is scheduled for 18 - 20 weeks. I couldn’t tell from your post about how long she’s been claiming to be pregnant. If she has irregular periods, she might not have had a pregnancy test until even 9 or 10 weeks, which seems possible if she wasn’t trying to get pregnant and had no particular reason to think she was pregnant. She says she is having twins “a few” weeks later, so is that maybe 2 or 3 weeks later? That could put her at the 13 week mark, which is early but eh, a little possible? There are also other tests a pregnant woman might have that reveal gender that would happen at the same time as an US, so perhaps she’s not quite understanding a gender result coming from the US itself or from a CVS test (done at 10 - 13 weeks, and you are literally getting an ultrasound while they do the CVS, so I can see how someone could end up with the impression that the gender was determined by the ultrasound). I find it a little surprising that her doctor hasn’t given her even an estimated due date, based on the development shown in the scans. In my expert opinion as a stranger on the internet, this is what raises the flag for me. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [December 7, 2015, 7:20pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/7 "2015-12-07T19:20:09Z")
**Twins: at what point in gestation can sex by determined by US?** I knew that the National Security Agency was doing a lot of electronic intelligence gathering on behalf of the US government, but this really does seem excessive. 😃 ---
**Author:** ![Blackberry](https://avatars.discourse-cdn.com/v4/letter/b/ecd19e/32.png) [@Blackberry](https://boards.straightdope.com/u/Blackberry)\ **Post date:** [December 7, 2015, 8:20pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/8 "2015-12-07T20:20:24Z")
> [@delphica](#): > > I find it a little surprising that her doctor hasn’t given her even an estimated due date, based on the development shown in the scans. I find it completely surprising and would be very surprised if it were true. Lots of women have erratic cycles but, if they’re under medical care, I’ve never ever heard of one not having a due date. ---
**Author:** ![Maggie\_the\_Ocelot](https://avatars.discourse-cdn.com/v4/letter/m/df705f/32.png) [@Maggie\_the\_Ocelot](https://boards.straightdope.com/u/Maggie_the_Ocelot)\ **Post date:** [December 7, 2015, 8:38pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/9 "2015-12-07T20:38:06Z")
So my friend Karen was a heavy girl - she was losing weight, successfully, but it was all coming off her hips and thighs…one day she looked in the mirror and was like, holy crap, I almost look pregnant. And she was - about 6 mo along. Erratic cycles and all. Then a month or so later the doctors took another ultrasound and were like nope, she’s almost 9 mos. Then she developed pre-eclampsia so (since the baby was so far along) they did a c-section - and it turned out that developmentally, the baby was about 7 1/2 mo - the first estimate had been correct, and the baby needed to be in ICU as she was a preemie. So it’s very possible for the doctors to be unsure as to gestational age, if the woman has had irregular cycles and didn’t know she was pregnant for a while. And Karen didn’t ever really look pregnant, she just looked like herself. On the other hand, this sort of situation is pretty rare, and with Karen the doctors at least were _estimating_ how far along she is, they weren’t just shrugging it off, even though they were wrong. So it’s entirely possible that the woman at work is lying about being pregnant, and will have a “miscarriage” in the next few weeks. ---
**Author:** ![eclectic\_wench](https://avatars.discourse-cdn.com/v4/letter/e/ac91a4/32.png) [@eclectic\_wench](https://boards.straightdope.com/u/eclectic_wench)\ **Post date:** [December 7, 2015, 9:20pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/10 "2015-12-07T21:20:25Z")
A friend of mine found out she was carrying twins very early on - 6 or 7 weeks, I think, so wayyy before she was showing. She found out she was pregnant, started bleeding a few days later, thought she was miscarrying, went in to get checked out - and not only was the heartbeat there, so was the other heartbeat. On one pregnancy we found out the sex at the 20-week anomaly scan. On the other one, I remember one of the early scans checking for sex (12 weeks, maybe?) and not being able to tell because the baby was in the wrong position. I don’t know how sure they would have been otherwise. When people show varies hugely. The standard wisdom is that fat women and tall women show later - but I’m not fat, I’m downright short, and I didn’t show till around 6 months either time. (Then in the ninth month I turned into a beach ball with a head.) The due-date thing does sound weird. On both of my pregnancies, the early scans gave a very specific gestational age (like ‘8 weeks 3 days, +/- 5 days’ type of thing), so even if I’d had no clue about my last period, they could have estimated the due date to within a week or so. As far as I know, later in the pregnancy it’s harder to be accurate, but early development is very predictable. But it’s possible that she just doesn’t want to tell anyone the due date. Some people are weird about that. And with twins, the due date is even less likely to have any relationship to when the babies actually arrive, so more reason not to tell people. ---
**Author:** ![even\_sven](https://avatars.discourse-cdn.com/v4/letter/e/dfb087/32.png) [@even\_sven](https://boards.straightdope.com/u/even_sven)\ **Post date:** [December 7, 2015, 10:02pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/11 "2015-12-07T22:02:54Z")
I don’t know about ultrasounds, but there is a relatively new test that can determine sex through fetal DNA in the mothers blood at around 12 weeks. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [December 8, 2015, 12:49pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/12 "2015-12-08T12:49:35Z")
Wow, this is all very interesting/informative. She is very nice, if a bit strange, and I hope all goes well. ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [April 1, 2016, 5:56pm UTC](https://boards.straightdope.com/t/twins-at-what-point-in-gestation-can-sex-by-determined-by-us/739713/13 "2016-04-01T17:56:29Z")
Two healthy baby boys were born 3 days ago. Mom and babies doing fine. She went from not showing to being HUGE overnight. She continued working; tended bar the day before going into labor.