@Volcanicorange
Falling pregnant so soon after your last pregnancy increases the risk that you will be deficient in important nutrients. For instance, you have a greater risk of developing iron deficiency and anaemia (Morasso et al; Vandenbroucke et al) and gastroschisis (Getz et al).
-You have an increased risk of placental abruption (Blumenfeld et al).
Compared to women who wait two years to conceive, you are 3.4 times more likely to experience labor or delivery complications (MDCH).
If your previous birth was via caesarean, you are at an increased risk of uterine rupture if you attempt a VBAC. To give you the stats: waiting less than 6 months before trying again will triple your chances of uterine rupture during VBAC (Stamilio et al; Esposito et al 2000).
-Your baby is at a greater risk of having a lower-than-expected or low birth weight (less than 2500g), being preterm (King). This is thought to be linked to the low collagen concentrations in the cervix of women with closely spaced pregnancies (Sundtoft et al).
-They are also at increased risk of suffering from congenital anomalies (Chen et al), schizophrenia (Gunawardana), menstrual disorders (Smits et al), and low IQ (Probert; (Pettersson-Lidbom and Skogman Thoursie). The latter is particular so if your child is a boy.
Your baby is also at an increased risk of stillbirth or succumbing to early neonatal death, even if you live in a high-income country. Ouch! (Wendt).
You are more likely to develop the baby blues (Gürel).
The shorter the interval between pregnancies, the higher the SIDS rate (American SIDS Institute).
Over-supply of breastmilk is more likely to occur if your children are closely spaced. It’s as though your body is producing milk for a bigger baby and your newborn struggles with the faster flow of the milk (Cave and Fertleman).
-Looking after a baby while you’re pregnant can be very tiring. Your body won’t have fully recovered from the last pregnancy. You may be excessively tired and easily run down. Iron and calcium stores will not have had time to replenish (Winkvist et al; King).
-Anything less than an 18 month gap has been shown to reduce a woman’s life expectancy (Centre for Population Studies; The Independent).
-Some studies suggest that you will be more likely to mistreat your children if you have spaced them this closely. 18% more likely, to be precise (Thompson et al).
-Having a gap of less than 17 months is associated with a significantly increased risk of having a baby of prematurely and underweight. The risks are highest for babies conceived less than six months after the birth of a previous child (World Health Organisation; Conde-Agudelo et al).
-Your children will be at an increased risk of attachment problems. The theory is that neither child gets enough attention from the mother to create the close mother-child bond that children need to flourish (Kauai Longitudinal Study).
Common interests lead to competition and one of the children (usually the youngest) can become insecure if he constantly does less well than his older sibling.
-It’s not all good news for the first-born either. Because their sibling was born before the older child lost the belief that they’re responsible for everything that happens, a long-last psychological legacy often occurs: essentially, the older child, driven by fear of rejection, will have a chronic tendency to be highly self-critical and less likely to forgive themselves when they make mistakes. Many firstborns never lose this tendency to feel guilty and/or overly responsible when things go wrong (Blair).
-Some recent studies suggest that children who are born only a year after an older sibling are three times more likely to be diagnosed with autism (Gunnes et al; Cheslack-Postava et al). This is because women are more likely to have depleted levels of nutrients such as folate and iron, as well as higher stress levels, after a recent pregnancy (affecting fetal brain development).
Many studies do not explain other causes and reasons for links eg. deprivation and poverty, malnutrition prior to initial pregnancy, mothers history of parenting by her own mother, access to contraception and abortion, genetic history of autism and associated disorder, lack of helpful advice and support for mothers etc.
Having a sibling with attachment disorder myself (sibling and I are adopted) I can tell you it would not go unnoticed if almost every set of twins or close in age sibling sets had attachment disorder. Equally, I know several siblings born close together with great parental and sibling relationships. Attachment disorder is related to secure attachments not being formed in early life, this can be caused by many things including needing an incubator and not having bonding opportunities due to medical need (especially in years gone by, nowadays they try to support relationships and bonding, including cloths smelling of mum etc), being moved from one family to another, having several caregivers (eg. Some children would be cared for by nurses on the ward whilst awaiting a foster family) as well parents who consistently do not respond to babies crying, do not engage baby in play and eye contact or cruicial touch (stroking, patting back, bathing, skin to skin). If a mother struggles to cope or has mental health issues that affect her ability to bond and respond, a baby may develop attachment problems, this can occur with no siblings at all. It is not the sibling that makes the difference, but the parenting and the support that the mother receives in coping with two young babies. The parenting she received also comes into it, as does her own beliefs and expectations of her children and the experience of motherhood.
Sorry, but that study just seemed a bit bonkers. It doesn’t really give a picture of how big an issue attachment disorders are when someone has them, for them and their family, and it certainly gives a wishy washy reason for them. They’re really about trusting your caregiver will and can respond to your needs correctly and lovingly, or in the case of a disorder - your experience being that they won’t. Your experience of the world as a baby is your caregiver, so if you learn that you can scream but no one will come (Or maybe someone shouts at your or hurts you) and you still stay hungry/hurting/wet, you stop screaming and believe the whole world will behave in the same way to you. Having a loving mum pop you on her other boob while your sibling takes the other does not cause this terrifying view of the world and other people to develop.
Anyhow, YANBU OP. GP should have admitted they weren’t certain. They should know basics like this, but then i was shocked when I gave birth with only paramedics who turned up halfway through and had never seen a birth! Had no experience but a paragraph from a text book, didn’t tell me when head was coming and kept trying to move me though I was 5 minutes from crowning. Any medical person not experienced in labour, breastfeeding, newborns and babies needs to check if they’re uncertain really. It isn’t good to give out incorrect advice.
. This was 26 years ago, I'd have thought GPs would be better informed these days. 
